There are ways to encourage Nullsec entities to limit sovereignty to systems they are actually living in that don’t involve a lot of new space. For example making TCU & IHUB Citadel service modules that consume fuel would change the economics of large empty empires.
I’m not sure it would encourage a lot more players to move to Nullsec - I believe the people who enjoy that playstyle and want to live there already do. Small alliances that aren’t strong enough to stand alone can join the existing coalitions.
The last time CCP showed us the numbers, 70% of players lived in highsec and I doubt that has changed. They may be casual or social players who simply want to enjoy flying Internet spaceships without getting involved in the metagame. They may play the economic game and want to be close to markets - whatever. If the existing 8000 mostly empty systems can’t encourage them to move I doubt adding thousands more would make much difference.