Sites can potentially be added to the list if there's a clear need for them. We can see if there's really a need by looking at the stats for the past 90 days.
In the last 90 days there have been 8 questions successfully migrated away from this site.
- 6 to Graphic Design (and 1 rejected migration)
- 1 to meta.ux.stackexchange
- 1 to workplace.stackexchange
Now, (in my opinion) that's not really enough to necessitate a change to the migration process. Currently, if a question should be migrated then it can be flagged for moderator attention and it will be looked at by the moderators and decided upon, which seems to be working pretty well.
One of the issues with migration is that people will regularly suggest sites that aren't at all suitable (usually if people mention anything to do with development they'll be told in comments to post it on StackOverflow, and 99% of the time that's a terrible suggestion. SO is a ruthless site and most questions posted here would in no way be suitable for posting over there. So if the question gets migrated to the wrong place it'll get loads of downvotes, negative comments and will just generally demotivate the poster from contributing to the network as a whole.
There's a bigger post about migration over on the main network meta page: What is migration and how does it work?