We are semi open to immigration. What should we do make it fully open?

When a state is semi open it means that only bases below 1M in immigration rating can enter. Any level base can leave though. Once it is full open any bases can enter.

The status is affected by the Province strength which is based on the top 100 players levels. Once this gets high enough then the immigration status will become full open. So your own states players need to be stronger before the game will let more strong players arrive.