Since the DC already scales directly with your BAB, it goes up pretty quicky on its own (when compared with most other special effect DCs that only scale by 1 per 2 levels, if ever), I don't think Paizo will see the need to provide an upgrade to the DCs.
You may, of course, ask your DM whether he allows to take the Ability Focus Feat (which would increase the DC by 2). Not exactly RAW, but I could see it happening.
RAW nothing prevents you from taking monster feats, and you could certainly apply ability focus to one of those feats. Some DMs don't allow bestiary feats, though.
Don't know of any other way to boost them, though. If only an ability modifier were factored in...
RAW nothing prevents you from taking monster feats, and you could certainly apply ability focus to one of those feats. Some DMs don't allow bestiary feats, though.
Actually, the problem I see is not with Ability Focus beind a Bestiary Feat. Problem is...
Prerequisite: Special attack.
Benefit: Choose one of the creature's special attacks. Add +2 to the DC on all saving throws against the special attack on which the creature focuses.
Those feats for upgrading critical hits are not exactly 'special attacks' in the sense of the wording of the rules, so, IMHO, they are not eligible for Ability Focus by RAW. On the other hand, I wouldnt consider it imbalanced. Thus my statement "Not exactly RAW, but I could see it happening."