It's never made sense to me that it's as easy to Tumble past (or through) a blind kobold as it is to Tumble past a Great Red Wyrm;
Because all tumbling does is keep you from
opening yourself to an attack on your round. Attacks of opportunity are based on the defender, not the attacker (with the exception of the Rogue's opportunist ability) -- you might as well ask why it's as easy to cast defensively against a kobold or a Great Red Wyrm.
(of course, it isn't, because if you fail the Wyrm has a much better chance of getting you, for more damage, then the blind kobold. But that's true for tumble as well.)
Long story short (too late): what are some good House rules for "fixing" Tumble?
Now that I've gone off about how it's not broken
here's some thoughts about how to make it scale...
- Convert it to a dodge bonus of some sort versus the AoO... Maybe a +8 on success, with the same DC to move through an opponent's space.
- A Reflex save versus the Tumble check to block movement through an opponent's space could work, but I wouldn't make that an auto-AoO (unless used in conjunction with the dodge bonus)
- Or, just make the DCs variable... 10 + Opponent's BAB to tumble, 20 + Opponent's BAB to tumble through... (Not sure if those numbers are scaled appropriately... Adjust as needed.)