Are the number of migration paths one-size-fits-all?
Server Fault is in a somewhat unique situation as far as sub-communities go (Super User may be in the same boat, but I rarely spend time there any more). Area51 has fractured our community into many smaller, but more highly specialized communities. We have: DBA, Webmasters, Unix, and Security that were all once handled on Server Fault and now have specialized communities. To a lesser extent, some content on Drupal and SharePoint might also have been on-topic for Server Fault at one point, but now has its own community as well.
I've also heard from various Server Fault mods that The Powers That Be have decreed that there will only ever be 4 migration paths. I believe there was also at least one m.so post that's authoritative on that as well.
I think that 4 is a good number for most sites. Bicycles, Gaming, Cooking, English, etc, will likely never need more than one or two migration paths. Four is probably even overkill for them! I feel that Server Fault and possibly Super User are on the opposite end of that spectrum. We have so many sub communities that have split off from us that having more than 4 migrations paths would be both beneficial to users asking the questions and users keeping the original sites tidy.
I don't think that all SE sites having the same number of paths makes sense, especially at the rate that Area51 is popping out new sites with overlapping content.
TL;DR - I don't think that the number of migration paths are one-size-fits-all
Arguments against adding additional paths
When I've brought this up in the past I've heard a few arguments against adding targets that I'd like to preemptively address:
Only a small percentage of questions are migrated to sites outside of the migration path. It's not really needed.
It's not fair to assume that users will flag and type out an explanation of why something should be migrated somewhere in the path (and risk losing fw) when they can just close it outright as off-topic. There are a lot more users on SO, so I'm sure this isn't a problem for you, someone will likely flag something for migration to the right site. On SF, that's just not the case.
If you increase the number of options, the percentage of questions migrated to those sites will increase.
People are stupid! Too many migration paths will confuse new users!
The threshold for voting to close is 3k. Users should have a good grasp of things by then. I do agree that this could potentially open the door for an increase in "bad" migrations, so I propose a tiered voting system: At 3k, you can vote for the 4 standard paths at 10k you can vote to migrate to a larger set of sites.
This should allow most questions that belong elsewhere to get there safely without much downside. By the time a user hits 10k, he or she should have a good grasp on how we work.
Where do we draw the line for what sites should and should not be in the migration path? It seems like a slippery slope!
Well, it is a slippery slope, but it's one that we're already on. With the success of Area51 and all of the sites that it's launched, we're facing this problem already! Server Fault gets a lot of Unix questions, because people assume we're the best place to ask (and we were at one point), but if they're not System Administration related, we don't want them any more. Times change, we need to keep up!
Area51 is a great thing. We need to make sure that use old foggies from the original trilogy sites can keep up with the times. Adding relevant migration targets helps promote these new communities as well as keeping the original site on-topic.
I think that if a general consensus on each site's meta is that a site should appear in the migration path for 10k users, then we should make it so.
Pls let us haz moar sites in da migrashun list. Even if it's only for 10k users.
I know that The Powers That Be have been adamant in the past about the number of migration targets in the list, but I think that it really needs to be reconsidered in light of the success of Area51 and the new communities that overlap with us old-timers from the original trilogy.