I just remeberd one of the best power creeps i liked a lot. Dawnforge: Crucible of legends by Fantasy Flight Games. All the races had racial feats and racial transformations which you gained as you leveled up. There were some cool options there.
Even official WoTC catalogue is massive, but when one adds all the 3pp stuff, it gets even more overwhelming.
For new GMs, best way to get into 3.5 is starting with core only then slowly introducing splatbook options. And while 3.5 had some absolute bonkers combos and rewarded system mastery, for me and my groups, real fun was playing with bunch of tier 3-4-5 classes. They were weaker, had situational abilities or were very niche, but they were fun. Also, game was much more balanced and actually playable at levels above 10. Classes like Warlock, Swashbuckler, Shaman, Shugenja, Marshall, Ninja, Scout etc. None of them are on par with those tier 1-2 classes, but when you make party of those classes, DM can breathe more easily.