you don't need to spend MP to expand though. the main opportunity cost of expansion is cash and manpower, with a trivial amount of points spent on war exhaustion, peace terms, generals and the infrequent non-vassal-annex manual coring.Another concern:>
Please consider looking at buildings. In the Tall vs. Wide thread it was basically established that tall empires are basically useless. Acquiring more territory is vastly superior to improving the homeland. If buildings were both more expensive and more powerful there would be a strategic choice in spending MP's to expand versus spending them at home on infrastructure.
It would be nice if a 20 province heavily upgraded european power could compete with a 50 province no infrastructure power for example. Englands approach to its colonial empire is another good example.