Interesting. We don't normally see this until such games are very old. Equally EA seem contend enough to do it every year for people so there is also that, unless this is one of those times where they screwed up later versions on the mechanics side of things.
Anyway that is a wide variety of things you want to do there, some of them on the more advanced side but still within reason. I also don't know how exotic they got with things here -- if every player is their own model these days rather than generic kit for team (home/away/goalie versions I presume), skin colour, hair colour, name/number on the back tweaks like it might have been for older games then that would add a further layer of complexity, not insurmountable but potentially a lot more work or some compromises. To back to the annual sports game release thing from the opening line if it is going to be unique models of players then many times the formats will be similar enough between everything that you can possibly backport existing content from newer games, though that does make copyright a bit tricky.
I don't know what the commentary thing is going to be like either, hopefully you don't have to get to the point of finding a voice double, importing newer things or busting out the deepfake voices thing*.
Kits is probably going to be texture modding, and maybe some stats aspects for the numbers/names. I don't know whether face scanning is employed in current stuff. Equally I hope the textures don't have displacement data -- while textures of old were 2d images wrapped (hence terms like texture unwrap) around a model, said model maybe having a slider for height/weight or multiple models for various body type ranges then modern textures (not sure what goes for the Switch) can also have displacement to make the textures stand out further from what is then a simplified model.
Players is probably going to be in the stats aspect (don't know what they do for this compared to football management stuff) and possibly some texture stuff. You might be able to find the stats table manually (find stats in the RAM, search the ROM, names/extensions/directory names, as a consequence of
cheats...) but it is one of the more annoying things to reliably find without delving into code. If you are delving into code then chances are the players have stats for whatever is on the screen and probably a few hidden ones as well -- something graphically or in the virtual game space (graphics are just graphics a lot of the time and the internal state of the game is something else entirely as it simplifies the maths) will be responding to something that speaks to the internal stats eventually so work backwards from there with a debugger. If it is the sort of game where you can train people up to be better at something then there is that too.
Stadia are more likely to be models editing, though they might have a generic one and slap a different skin on it to bring it back to texture stuff. Also if it is going to be like modern advertising on things in modern football where I see they have animated advertising compared to the static/maybe rotary stuff of old/poorer teams then you might also find yourself playing with animated textures.
There are some tools to rip Switch textures and import them into other things. Hopefully EA or whoever makes these games these days stuck with those rather than rolling their own format. If not then you get to have fun with that too.
*choice videos
If you are new to all this and that sounds like a nightmare (it sounds like one to me and I typed all that from without reference to anything) then I would suggest a simpler take. Take one team and bring them up to current team -- if someone changed position then so it goes, if they got fat and slow in their old age (or maybe more skilled) then that, if one of their youth team (don't know whether they are included by default) made the main then go that, if one of their substitutes made main player then I guess that, or transfer from another team rather than whole new player is available then figure out how to include them in this (maybe also fix it so the player is not cloned by virtue of still being on the original).
