Apart from this, I'm concerned for the full game.
If it's Steam only, I suppose we will be able to edit rosters/names/appearances as they're stored in the EDIT file in the Documents folder (assuming paths/file formats won't change) but... Will we be able to edit kits/balls/boots which are in the .img files or, even more difficult I believe, the .exe to attach a kitserver (should one be available and should there be the need) or to alter gameplay (should there be the need)?
I know that with FIFA (which from FIFA 12 on is Origin-only if I recall correctly) is possible to edit .big and .bh files (and it also has a kind of "afs2fs" structure included) but I have no experience in modding Steam games.