Yeah, its weird there arent more character hacks or pallet swaps, but 2 player 3-vs-3 is a big ask for a GBA title.
There is absolutely no info on how to modify Tekken Advance: http://www.romhacking.net/games/1522/
I think 11 years after its been released is either perfect timing for a revival (if you are going to spend the time to learn how to make hacks) or way too late in the games life cycle for anyone to bother with.
I played this on an emu a while ago, wasn't impressed back then tbh.
just found this quote
This stripped down version of Tekken might turn away some of you for the obvious lack of the 3d depth on this version but it's still a great 2d fighter regardless. It features some characters from Tekken 3 (luckily my personal fav made it in the game) not all of the fighters made the cut but it's a GBA port so what do you expect? The difficulty is just right for this game you can adjust the difficulty to fit your experince you can make the game easy if you're nwe to the series or you can give yourself a challenge by switching to hard mode. Despite being stripped down the plethora of extra modes made it through. Survival, Arcade, Time Attack and even and brand new mode all made it in. The new mode is called 3 on 3 it's basically a portable version of Tekken Tag's Arcade mode but with a extra player on both sides. Even sweeter is the addition of a 2-player link up mode! So you and a friend can battle it out on the bus or wherever. Tekken Advance is a good port of the fighting classic and derserves to be part of your handheld collection.