Michael Phillips is the #1 choice for me overall. I think his combination of overall film knowledge, proven chemistry with Adam, and witty + intellectual personality is a perfect mix for the show. Unfortunately, I'd imagine that given his regular job at the Tribune and other appearances he makes elsewhere, his time is likely too scarce for a weekly podcast. But I LOVED the episodes with Adam & Michael when Matty was running off to do acting auditions (and nosebleed into popcorn).
For the females, Dana Stevens is by far my #1 choice. I thought she was great in her guest appearance, and is always really insightful and makes great arguments in her Slate podcast. I like the fact that she has a great presence on radio...kinda fills up the podcast room, which I think would jive well on Filmspotting.
Steve Prokopy would round out my top three choices. I think he would bring an interesting slant to filmspotting...like Sam/Adam/Matty, a guy who is a "self made" professional film critic. I like the fact he has the Tarantino-like film nerd personality going on, and that he has seen what seems like a scary number of films in his life, and that he'd bring a slight "niche" edge to the show, as clearly he has a love for tiny independent films, black comedies, foreign movies, and nerds out on the fantasy/sci-fi/horror genres.