Well, Lunatic High already grants Haste and Howling Moon grants that and Berserk (you can cure Berserk or avoid it by equipping him with an accessory). I think Blood Fang should grant a Power buff (or maybe curse type where his attacks are strengthened but he takes damage when he attacks).

Another thing about limits. I think that you should be able to reset the level of limits you're using mid-battle. It'd be convenient for those who want to avoid certain skills in battle like Red XIII's Earth Rave which a fire-elemental attack that can heal Fire-elemental monsters. It's less convenient than simply having all of the limits displayed on screen at once, but it's also less clutter.

LEVEL 4!
I think that acquiring Level 4 should be more interesting than acquiring a manual. I envision a top-score trophy in the Battle Arena still called Omnislash but serving as a Key Item that doesn't vanish from your inventory. Instead, acquiring the necessary points to earn it would set an automatic event in place where Cloud must face a Black Cloak in battle who turns out to be Sephiroth (clone). Winning gets Cloud the Masamune (replica) and the Omnislash limit break which he'll automatically use to defeat the Black Cloak. This will alter the very end of the game where normally you'd use Omnislash even if you had never acquired it. In this case, Cloud will use a new version of the Omnislash that was never available throughout the game. The Omnislash Battle Trophy would be given to the party automatically (at least preferably, otherwise you may forget). It would be great if you could fight machinations of Genesis and Angeal in Battle Arena and win replicas of their swords (of course, Angeal just uses a standard-issue sword) which Cloud could actually use.

I would still like to see Tifa have a flashback to learning from Zangan and failing repeatedly to learn the Final Heaven.

I would like to see a last conversation between Vincent and Lucrecia before he acquires Death Penalty and Chaos showing a new flashback that shows Chaos' original form (the one seen in the original game) before Lucrecia made it a part of Vincent. Rather than acquiring an item that enables this ability, I think Vincent should black out and awaken as Chaos (in neo-polygon form, like a Chibi-Chaos). Then he'll examine the Death Penalty as the scene closes and fades to black.

Also the battle with Lost Number should not be a requirement for recruiting Vincent. I think it should be battled later when Nanaki is ready and it should be a Gi fight.

Yuffie's All Creation should become available for use during her fight against Godo.

I think Cait Sith should have two more Limits. I thought up a Roulette wheel full of Buffs, Debuffs, Elemental attacks, Heals, Steals, Throws, a physical attack, and a Death attack which would each randomly affect a single character. I imagine Cait Sith's superior version of Heal would be like Angel Wings and can heal a character or resurrect a fallen character and his superior version of Throw would allow him to throw replicas of the weapons you have stocked but still deal close to the same damage. The item thrown (including Gil) would be random as well and he could even throw materia which would deal damage equivalent to its power and context (i.e. elemental damage, status damage, 2-4 hits for 2x-Cut). Steal would at times do random damage but if the random character selected was a party member, you'd get the message "What are you doing?"
As for his Level 4 limit, I thought up Black Cat. Instead of just growing big with the Moogle, Cait transforms into a black panther who sits and cleans itself (licking its paw and rubbing its head). The enemies will be put in Darkness status. While Cait Sith in his panther form, he can't be attacked even if both ancillary characters go down, but if he chooses to attack, the Limit will cease and things will return to normal. He can take other actions without affecting the limit though.