In the soul of chaos, which is exclusively on the GBA, is a dungeon that you go to after the game, and you get to fight the 4 guardians once again, ut more suped up, as well as some famous bosses from FFIV,V,VI like Ultros, Gilgamesh, Atomos, Doom Train, Shiryuu (optional dragon from FF V), and some others that i forgot the names of.

P.S- srry if some of this info isnt true because i just got this in the back of my head while i was typing