The spell system is probably my favorite thing about the game. That being said, I'd like the core book to be thin as well. One thing I'm not sure about is that I noticed all the tables are called out as product identity in the OGL of the Beta rules. Does that mean all the spells are technically not open?
Basically, as long as the spells were open content, someone could create a tool like this:http://www.thegm.org/perramsSpellbook_d20pfsrd.php
If such a tool existed that could create one-spell-per-page PDFs of 5.5"x8.5" size (or multiple formats for those who have other preferences), that would be all I'd ever need.
At first, I was a bit taken aback that only 1st-level spells were included, but I've grown to like it. I think the best thing to do would be to figure out a standard requirement for the "official" format, include a list of sample spells, and let the DIY community go wild with creating their own spells. But it would be important to have those formatting guidelines so no spell (rituals excepted) should ever be more than one page (or half-page). Then users can grab spells from multiple sources and not have to spend a bunch of time tinkering with the formatting.
I will say that I think the spells that are more than one page in the Beta rules are too much. It's gotta fit on a single letter-size sheet maximum
One final suggestion would be to have a free PDF spell supplement available for download, that would have enough basic SRD-type spells for those who don't want to bother with the custom spell aspect of the game. I guarantee some people will fall into this camp and would feel ripped off if they didn't have all the spells needed to support the stated level range of the game. And for guys like me, I'd hate to have my core book be 320 pages because of those types. I already have to deal with my 500+ page Pathfinder book, of which I actually use maybe 120 pages in play.