Well, my bosses have been a little hit and miss so far but I've got some notes for some mechanics that could turn into something interesting:
Three-Team Fight Super-Boss
-) One thing I noticed in testing with Bizarro was that a three-man fight is actually quite difficult if the Core has to be killed and requires a fair amount of planning, especially if one of the teams only has two people. What I was wanting to do was design a boss around it and set up three very different challenges for each party. Bizarro, the original, tends to have only a little variation in the attacks he uses on each different party but there's no reason the fights couldn't be wildly different from each other. It could even be three separate bosses that are linked to each other through Bizarro's AI.
General stuff
-) Before I released the mod, there was a boss that had a No. 24 (FF6) style 'Barrier Shift' that would change the boss' elemental weakness whenever it took a few hits from its current 'weak' element. I was too inexperienced at the time to get it working properly, but I've always been meaning to go back to it. Something like that could be handy here, maybe coupled with attacks that alter the party's elemental weaknesses as well (so long as this is telegraphed to the player that it has happened).
-) Related to that, something that caused a lot of grief on the NT Mod's Jenova Life fight was the opening 'attack' that sets the player party to nullify the Restorative element, cutting off conventional healing. While I'm planning to change that to 1/2 resistance for the 1.2 release, this mechanic is great for flipping a battle on it's head. Regen can derail it, but some (sparing) use of Resist, Reflect, and/or Dual Drain should be good counters to that.
-) There was a Hardcore Hack (Gameshark) fight where an enemy was reducing MaxHP instead of just Current HP with it's attacks. Replicating that in some fashion for FF7 PC is on my 'to-do' list. The video was here:
http://youtu.be/3-WBTbm5Vcw-) I've also been meaning to look into attacks that disrupt Limit gauges, or which deal damage based on how full/empty these are. I think from what I saw in the save editor that the Limit value clocks in at 0-255 and there's an OpCode for it so maybe-
-) Status Stealing could be a handy trick, by transferring status ailments/buffs from one target to another. A boss that can steal Barrier, Haste, or Peerless would be a pain, or if it were taking these statuses from 'nodes' that only use protective magic on themselves. Or something that can transfer bad status ailments from one character to another, regardless of resistances, could mix things up. Lost number has a bit in his AI to Transfer Status from himself to his next form but not sure if that'd work or not.
-) On that note, I was also wanting to see if an enemy could copy a character's stats and push them into their own stats. Would be handy for 'Mimic' enemies that use the character's model.
-) There's a flag for 'Invisible' which I think is used by Ghosts. It'd be interesting to see what happens if it was used on a player character as part of an attack, or if the player had to trigger this to dodge some incoming super-attack-
-) Adapting to Elemental Weaknesses: I think Karifean was able to get this working for his Super Boss Rush mod, where an enemy would refrain from using attacks that would heal the player and use a different one instead.
-) One thing I've been working with is ways to use enemies differently in boss fights. I had a fight where a Gigas-type enemy would appear with four Bombs and periodically the Gigas would attack the bombs which, if killed, would hit the player with a powerful self-destruct attack so the goal of the fight was to balance attacking with keeping the Bombs healed.