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).