Currently most sites submit their challengecount.
I know that rankk.org is not sending challenge count atm, but i think it will soon.
Tommorow i will check all other sites if they make use of it. (all others do that already iirc)
For the rescoring i was thinking about a formula too.
My idea for base-score was also something like challcount * difficulty (the computed solving difficulty), but it wont work very well i suppose.
For example i think that electrica, although it has only a few challenges, deserves a higher basescore than my above formula would calculate. Maybe rethink the difficulty formula ?
Your idea of expotential scoring on the other hand might work very well:
Quote from Kender
Is solving 20% on 4(typo?) sites worth the same as 60% on one site?
I think solving 20% on 3 sites is much easier than reach 60% on a single site,
thus i vote with yes for your formula.
Edit: I think the main problem is to adjust/calculate the basescores