A 30 level wizard with a bab of 15 is a problem because the lvl 30 fighter has +30 ? It's something I never understand with the ELH, because, as far as I know, a 30 lvl wizard use BAB only for touch attacks. The ones that ignore armor, natural armor and shield. Which, at lvl 30 is usualy a total higher than 15...
So, keep the BAB like it works in 3e !
Just add a magical BAB (at least one, maybe two if arcane and divine are REALLY different), wich will allow you to beat the defense of your target.
Saves are not that hard to fix : just like skills had only a x3 multiplier at first level, the +2 bonus of good save don't stack. Complete the system with a fractional save progression (rounded down), so that a rog5/wiz 5 has : fort +3 ; ref : +7; wil +7.