Not sure if this is been done, but I'm surprised it hasn't occurred to me before (or maybe it has)--but dear god someone hacking FFX to rebalance the final confrontations a bit so they actually feel like final bosses, especially
. Taking off the auto-life from the final sequence of fights I think could make things quite a bit less simple, especially if you've built up Yuna, but I think the other two should be quite a bit meatier and hit harder.
Or, if you really like the current balance, adding a trigger like in FF7 may be helpful. There, if you had Knights of the Round, the final boss had a bunch of HP tacked onto his total--something similar for FFX, but maybe including attack/magic/defense/speed as well as HP. If you hacked the remaster, I think killing just 1 of the Dark Aeons would be a good trigger, but in the original, maybe 1 fully unlocked endgame weapon should be the trigger (or maybe this would be better for both--Auron's probably going to do 50k plus per hit with his weapon, and IIRC his is pretty easy to unlock).
This way, folks who rush through (or LLG it) could still manage as ling as they don't trip the trigger, but those of us who do a bit more exploration would still get a fitting endgame fight.