I was thinking of changing some of the 3.5 rules back to 3.0.
Simple buff spells like Bull's Strength had random results (1d4+1) which hard unless you had a computerized character sheet. They also had durations in hours. Definitely liked the 3.5 version better. Again, Pathfinder continued the trend of shortening buff durations (tough not of these spells). Some cleric buffs gave you a static score (divine might giving Strength 18 IIR), encouraging min-maxing to the n:th degree.
I would probably end up just sticking with 3.5 as well, but I'd be sorely tempted to roll back to the 3.0 weapon size rules. 3.5/PF are definitely the odd-men out on this topic out of all editions of D&D. And I'm not sure that's for the best. I understand the simulative aspect of those rules, but I'm not certain they improved the game.
Haste in 3.0 made spell casters broken by granting an actual extra Standard Action. That meant two spells per round. In a party with any number of casters, that became the single best party buff out there. Spell casters were over done to begin with, power wise, particularly at higher levels. Doubling their power made them critically broken starting at 5th level.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.