We can't add more targets to the migration list (it's an arbitrary hard limit set by the SE staff).
The sites currently on the list (SO, SU, U&L, DBA) are much more important.
Some stats for migration in the last 90 days:
SU: 152
U&L: 79
SO: 37
DBA: 18
SEC: 4
When you think it's really necessary to migrate, raise a flag (the mod might disagree though and reject the flag).