The Panem map is based on essentially two categories: Things we know, because science (climate, land cover, fisheries, soil types) and Things we know because Collins said so (not that much, actually, but a few things).
I took penfold_x's map (based on fernwithy's map) as a starting point, and left her D1-D5, D8, D12, and D13 alone, because they make sense to me and I haven't thought nearly that hard about them. I think the main difference, philosophically, that I have is that I don't take the Mississippi river as a major boundary. Mostly because even if it was at one point, I don't think you'd give up all the excellent farmland in Illinois just because of a river.
(Forgive the Kid Pix-esqe map)
So on that note, let's start with the farming districts. What grows where depends, at the most basic level, on soils (which won't change much) and climate (which will). I assume that a) the climate is generally warmer in Panem than current North America, and b) a lot of the effects of climate change are mitigated by plant breeding and genetic engineering. One thing that's not likely to change is the gradient of increasing rainfall in the central part of the continent, moving east out of the rain shadow of the Rocky Mountains, and that's influenced where I've placed D9 and D10.
I put D9 on the best grain-producing agricultural areas in the US, and stretching down into the south for rice and cotton production (you could also put these in D11 but they're more grain/large-scale mechanized than horticulture/small scale labor-intensive these days). This could also stretch up further north into the eastern Dakotas and Canada's Prairie provinces, but D8 is in the way as of this map. If you want, you could put D8 in, say, Milwaukee, WI and stretch D9 northwest a good ways. I left it where it is in part because of where I put D6 (we'll get to that in a minute).
I assume grain agriculture is highly intensified and mechanized, so population density outside D9's main city is very low. The main city (which I put around modern-day Des Moines because it's central-ish) is where grain milling, ethanol refining and equipment maintenance occurs. I'm positing that Panem makes a lot of ethanol, probably cellulosic ethanol from crop residues not grain, because this is the future. Oil is going to be scarcer and more expensive so it makes sense to have alternative fuel sources. Note to rebels: At least at key points in the year (spring/fall), D9 has huge stocks of fertilizer (aka explosives). Between this and its status as key for food production, this district should be a priority.
D10 is further west, in the rain shadow of the Rocky Mountains. These are the drier parts of the Great Plains which are less productive for (non-irrigated) agriculture but good for grazing. I think cattle production is largely free-range in the southwestern part of the district, because there's lots of space, low population, and Panem needs grain for other things. Milk production would be slightly more intensive, but probably still relies on grazing. Hogs and chickens would be more intensive, and you can pretty much do that anywhere. I'd say it's probably in the northeastern part of D10 (intensive chicken production has heat dissipation problems in hot climates because of high animal density), near D9 to facilitate grain/manure transport between the two (and also manure transport to D11). Probably this would be in the form of pelletized chicken manure, which is already a thing you can buy and is easier to transport, although you could also fill tankers with hog manure and spray that on things (the "shit express" is usually your first assignment as a baby D6 railroader. 10-9-11 with manure, 11-9-10 with animal feed). Sheep might be up north near D8 so you don't have to transport wool that far. I don't know if anyone cares enough to have large-scale goat production (sorry Prim) but goats will eat anything, so stick 'em wherever.
I (like pretty much everyone else) assume the D9/D11 split is mainly D9 = grain production, D11 = horticulture, plus probably sugarcane. D11 is in the south, partly because Collins said so, partly because you can grow stuff there year-round. I assume things like coffee and cacao magically grow in the southern parts of D11 because Capitol genetic engineering plus climate change, and that since these are scarce and expensive, the crap coffee people drink in the districts is something mostly synthetic. Something like the depression-relic Postum my grandmother used to drink+synthetic caffeine. (Rokia drinks this because she finds it comforting and familiar, everyone else in Two, which gets better stuff, is like "that shit is GROSS how do you even drink it?") I draw it going as far north as central IN for those things like apples that really want an actual winter. Also because IN has very good soils and they'd be a shame to waste.
D7 is also natural-resource based and can be inferred from current tree cover (map) I see two possibilities that make sense for centering D7, one being northern Minnesota/Wisconsin/MI up through northern Ontario, the other the Pacific Northwest. I think it's likely that D7 crews travel around both those areas, actually, because there's different kinds of trees, different ecosystems, etc. So to make the Victory Tour less totally absurd with the zigzagging, let's say D7 central (at least for the purposes of the Victory Tour) is in Ontario somewhere. Then I imagine there's a long rail line that curves up through the northern forests and down through to Seattle-ish, for people and lumber transport. You could even have two hubs: one in Ontario and one in Washington State, for all we know, and they just use the one in Ontario for the Tour because it's convenient (here my map is similar to deathmallow's map)
I (surprising no one) have lots of thoughts about this district.
While I have very different geography for D6 than deathmallow, I did steal from her the idea that D6 mines iron and refines steel. Because someone has to, and it makes sense that the transportation people, who'd use a LOT of steel, would also produce it.
There are several iron mines in a band from the upper peninsula of Michigan to northern Minnesota, and the coal used to refine iron into steel would be coming from D12, so it makes sense for the steel mills and thus the rest of D6 to be somewhere in Michigan-ish. This has some obvious historical connections too--US Steel in Gary, IN, auto plants in Detroit, major railroad hub in Chicago, etc.
Then there's the question of whether everything is in one big city or sort of spread out: I have the smelting plants and foundries in one city and the rest of the manufacturing in another, just because otherwise that's a lot of factories and people to pack into one place and there's plenty of space so why not? I put the central manufacturing town (which is also where the inter-district train crews are based) in Chicago, the smelting up the East coast of Lake Michigan a little ways (yellow dots on the map).
Then the mining communities up north (red dots on the map) are small, isolated, and quite poor, a lot like D12 in canon. Traveling among towns in D6 is only possible with prior authorization, so for example, Rokia and her mom can move from their mining town down to the main city because her uncle signs papers saying Rokia's mom is going to work for him.
In terms of just what industries D6 has, I have automotive, hovercraft, and train production happening here, as well as major repairs. Minor hovercraft repairs the Peacekeepers and the Capitol can handle themselves, but major overhauls come to shops like Rokia's Uncle Salif's. There's a handful of mechanics' shops that get contracts for repairs of different kinds. These contracts are strictly Capitol-controlled and the second you step out of line you lose your contract.
D6 also provides the people who work on the inter-district rail system. These people have to have finished school to 18. Once they're past reaping age they get a few months additional training/brainwashing/loyalty testing and start on cargo trains to outlier districts (e.g. the shit-and-food trains I mentioned up above). It's only after years "proving yourself" that you can work on passenger trains and/or in the inner districts/Capitol. Contact between train crews and district citizens is also extremely limited and trains are spot-checked to prevent smuggling. But despite all this, there are train crews that do intelligence-gathering prior to the rebellion and sabotage once it starts--hassling Peacekeeper deployment, ensuring food gets to the rebels, etc. (Totally writing this. One of these days. I swear.)
There's also plenty of within-district divisions in Six, the way you have merchant/miner in D12, or quarries/cities in D2, or engineers/factory workers in D3. In general, skilled labor (mechanics, etc) = working on passenger trains > working on cargo trains > working on intra-district trains > working in manufacturing > working in smelters > working in mines.
And then there's the D6 drugs issue: it's more fanon than canon that D6 is the morphling district, apart from the D6 victors. I've picked that up to some extent. Increasing levels of mechanization seem to be a natural evolution of industrial economies, which means there's some level of structural unemployment, which among semi-skilled people with access to sensitive production environments could be dangerous. On the other hand, addicts are too busy looking for their next fix to worry about overthrowing the government, so I think the Capitol would turn a blind eye to relatively high levels of drug abuse in Six as the better alternative. Plus there's a combination of ways the drugs could get there: smuggled from the trains transporting them, home-brewed from the industrial chemicals floating around, or siphoned off from medical treatment of the likely-common injuries due to industrial accidents. I see it as something on the level of Detroit or the sketchy parts of Baltimore, enough to make the city dangerous and dysfunctional (and to have the reputation) but not everyone, obviously, can be a morphling addict and have the district still function.
AND THERE IT IS. Most of it's open to reinterpretation, but now it's at least written down.