the way to create a stable D2 is to say to D2 that promotion to MLS will be possible if certain benchmarks are met. Otherwise I don't see the lower leagues ever becoming very stable. There's just a ceiling on how much value a club that is lower tier can be worth when it cannot be promoted. You could put an economic trigger in that once D2 met the benchmarks, for instance 3/4 of clubs that play in a SSS, promotion to D1 would be go into affect within 3 years. It would probably take 7- 10 years just for D2 to get to that point.