Disclaimer: Hi! I'm one of the community managers at Stack Exchange. I'm sure you've seen some of us and a few developers around over the last couple of days, sorting out last-minute issues and problems arising from the site upgrade to SE 2.0. Welcome! We're happy to have y'all here.
Far as migration paths go, we'd be happy to set up up to 4 defaults for you guys, but there are a few considerations we usually take into account.
First of all, migrations paths are only available for graduated sites. This means that sites like Academia, Computational Science, Computer Science, and Philosophy are currently not an option for user-driven migrations. This is done to minimize the chances of newer sites and/or sites that have yet to cement their long-term success from being potentially overwhelmed with effectively rejected question from other sites in the network. We ideally want every beta to form its own voice.
In a pinch, diamond moderators have the ability to migrate a question to any site in the network (so long as it's not in the private beta stage), but this is ideally only done for exceptionally good questions and not just anything that could conceivably be asked somewhere.
Secondly, Mathematics is an obvious choice and it's already enabled, but beyond that I would like to suggest letting MathOverflow run as is for at least a few weeks to see what sort of questions you keep getting and what sites they get migrated to. We collect statistics on how many questions are migrated, where they end up, and whether or not those migrations are rejected by the destination site.
(Aside: migrations are considered to be rejected by the destination site when migrated questions are closed as anything other than "duplicate" or deleted. This is another flood prevention mechanism.)
In a few weeks/months, we all will have a better handle of what migration paths make the most sense and are most successful. In the meantime, you can request a migration by flagging a question and explaining what you'd like to happen, and a moderator will make the call whether to move it over to another site or not. We'll be happy to set up user-driven migration paths when we see clear patterns for migrated questions both in terms of quantity and quality (i.e. low rejection percentage).