Hacking for non-translation GBAFE projects really started in earnest probably around 2005, 2006ish if I remember correctly? Translation projects for FE6 and older predate that though. By that point, there was enough of an audience to try fiddling around with the GBA ROMs, editing stats and graphics, etc. Events would come later once they were figured out, the tutorial events in FE7 were disabled, etc. Once those hurdles were surpassed, then you started getting projects that weren’t just following the vanilla game structure because they were forced to do so.
Raw hex editing followed by the Nightmare 1.0 and 2.0 modules, to my knowledge.
If @Celice is still around, maybe they’ll remember more things in detail than I do? They predate my involvement in the community. It’s been a very long time and my memory is definitely full of holes since I was not overly involved in actually doing things that weren’t just graphics until later.
I know for GBAFE there was research into how the portrait graphics were aligned that VincentASM did and they did one of the early tests of importing brand new portraits into the games, NomadicTrooperGirl and others were able to independently figure out editing CGs around the same time that Blazer did, Hextator was instrumental in figuring out the beginnings of custom battle animations (beyond editing over the existing data and being constrained to the existing animation frames and slots), but I don’t know much about the early efforts for the translation patches (or any of the other hacking endeavors aside from FE404) aside from some of the old memes that used to float around IRC during FE6’s original translation effort about having to recompile the .bin files and reinsert them back into the ROM for the opening intro text and the turn counts at the end of the game and stuff (because the TL team member that was doing them never wanted to actually put work into doing it, despite the daily nagging).