So I was looking at the Combat Maneuvers, checking to see if performing a CM still provoked an AoO. I was wondering why the generic rules didn't have a mention of it when each maneuver basically says: "If you don't have the Improved XXX feat, you provoke an AoO".
I realised it was probably because each "Improved XXX" feat has a different name, which got me thinking:
Why not treat CM Proficiency like Weapon Proficiency? Make the base DC = 11 + opponents CMB. Then if you don't have proficiency with the CMB you're attempting you take a -4 on your CMB (effectively making the DC 15 + opponents CMB) and you provoke an AoO.
This could lead on to all sorts of flow-on effects like CM Focus/Specialisation, and making stunning strike a CM and hence the Monks bonus feats being "choose one CM proficiency feat", etc.
Also (this was discussed in another thread, but I'll repeat here) Weapon Finesse could become something like:
You may choose to add your Dex bonus (instead of Str) to your CMB for defence. Additionally, you may choose to add your Dex bonus (instead of Str) to attack rolls with any weapon of CM with which you are proficient.
Admittedly it isn't quite as backwards compatible - GMs will have to convert, eg: Improved Grapple to CM Proficiency (Grapple), on the fly but it does standardise and streamline the system nicely.