Things you think should have been in 3.5's core book?

[Nanashi]

I'm looking for non-core options that are straightforward, useful but not OP (ideally enabling new builds instead of making existing ones better), and ideally reprinted in multiple books. Off the top of my head I can think of Practice Spellcaster, Able Learner (Those two Neverwinter Nights 2 actually included in its core), tactical feats, the Twilight armor property. Any other suggestions?

[Stratovarius]

Tome of Battle.

[Raineh Daze]

That bit that had to go in every later book about immediate actions/.

[Keldar]

Psionics.  Its status as an add on caused innumerable problems, both among players and in development.

The Polymorph changes.  What a fustercluck of layered rules changes.   That wouldn't have been necessary if they had actually listened to the feedback about polymorph from Min/Maxers in the first place.  But not having to refer to at least four books to use one spell would have been nice.

[Nanashi]

How could I forget Improved Toughess? That one was reprinted three times and is outright named in a way to tell you the original version was terrible. It's how Toughness works in Neverwinter Nights II (and Pathfinder).

[Stratovarius]

Neverwinternights I as well.

[awaken_D_M_golem]

tighter math on Skills

(more) functional Stealth system

Warlock , Psi-something , maybe Artificer (or -ish)

the Warlord they hadn't really thought of yet, but party buffing has always been a thing, perhaps this is Functional Gish territory

Prc's that function as missing class features, for casters CSW