I was thinking this too...how cool would it be to allow all the template companies to submit some basic templates for inclusion in the stock install. Either they could be voted on (to keep them to a minimum) or heck, they could all be installed on a server somewhere and the user could choose one or all of them during the install-to be downloaded and installed on the fly.
This gives them all some exposure and at least gets some nice templates available at install (but won't bloat the install files). You could put together some sort of group who approves them (and make sure it's not weighted toward only the big guys). This little suppository of themes could grow over time, but be vetted so that it's not full of crappy junk that will break Joomla.
Something to think about