well the best way for me was to liberate a system most of time they have a new mission (specialty rts ones
)
and changing difficulty seems to give u harder missions if u chose easy difficulty there is less chance to get harder missions (again its very true for rts one)
another thing you can do is changing day every few days ( i am not sure maybe every month) u get a different mission
well the last thing is liberate different race and try your luck