I agree that INT is definitely poorly implemented in the game;
- There's only 1 INT based class (And 2 martial subclasses that use INT as a casting stat) so it's less useful for multiclassing.
- INT based skills are rarely useful and even when they are... They can be replaced by CHR or WIS skill checks for classes that use those stats.
- INT as a stat provides little passive benefit (Like how DEX provides AC, WIS saves are quite common from spells, STR is useful for jumping and carry weight etc)
I'd posit one of the better ways to try and help out INT would be to introduce the Artificer class. It would be another INT based class meaning INT would be useful for playing that class and multiclassing with it.
You could try to toss in more INT skill checks (And specifically make them INT skills only and not cheap out and let Sorcs/Bards CHR their way through them...) with more impact, but this might be awkward to implement and difficult to do in a way where people wouldn't just end up pulling Gale out of camp to pass the checks and instead actually invest into INT.
