First of all, I'm not sure how Jason Schwartzman's performance in Rushmore could be hated…he was brilliant, and so was Bill (and so was that movie). But to each his own opinion, I guess.
Second of all, I really don't see Schwartzman as an Egon-ish character. I don't see him as any kind of character from the films. Not to say that he couldn't bust, but…I dunno.
I also agree that it might be interesting to see different kinds of characters in the next movie. Then again, you ruin the chemistry of the original two movies, and GB3 becomes a failure with many Ghostheads. (Ex: SW Ep. 1, TPM was a failure to many Star Wars fans, because it didn't really follow any chemistry from the original trilogy, which is why AOTC is supposed to be better. I personally enjoyed TPM, but even though I have yet to see Ep. 2 and 3, I know that TPM will likely become my least favorite of the 6). I digress.
Schwartzman is a funny actor, but I don't see him as an Egon, and I have a hard time seeing him as a Ghostbuster. But I could be wrong.