Somebody recently posted a very theoretical question, and I went to flag it by clicking Flag > It doesn't belong here > off-topic, and I was unable to directly select cstheory.stackexchange.com. The cstheory site is where I felt it best fit, so I had to just flag it as is off-topic. I suggest that cstheory.stackexchange.com be added to the list of off-topic SE sites.
Unfortunately, sites aren't allowed to have more than 5 migration paths, and SO is already at the limit. To add cstheory another target would have to be removed, and I think all the current targets are used quite a bit.
Note that mods can migrate posts anywhere, so for cases like this you can flag for mod attention and ask them to migrate to cstheory
The flagging dialog supports flagging as "belongs on an unlocked migration path" but indeed, it does not support flagging as "belongs on arbitrary Stack Exchange site".. this is for a reason.
However, if you feel strongly a question belongs on another SE site, feel free to flag as
- it needs ♦ moderator attention
- this should probably be migrated to