I don't know why USL/NASL would settle for Division II?
I know some will hiss and snarl at this but there's no reason they can't rise up and challenge MLS aside for FIFA/USSF mandates but I could see US courts getting involved and telling FIFA to shove it.
A league of the best from USL and NASL would be very formidable right now, I think.
1. Indy Eleven
2. San Francisco Deltas
3. FC Cincinnati
4. St. Louis SC
5. Tampa Bay Rowdies
6. Jacksonville Armada
7. Arizona United (Now Phoenix Rising SC, terrible name!)
8. Pittsburgh Riverhounds
9. Sacramento Republic
10. Ottawa Fury
11. Louisville City
12. Carolina Railhawks
13. Miami FC
14. FC Edmonton
15. Nashville SC
16. Oklahoma City Energy
17. New York Cosmos
18. San Antonio FC
There are no FIFA/USSF mandates which prevent a competing Division 1. If all of those teams could get together, develop a consensus on a viable business model, implement it, and meet the reasonable USSF standards for a division 1, they will be division 1. Even if they did not get D1 status, that would really have no impact on their ability to challenge MLS, as the divisional standards have no salary caps.
The problem is, pretty much all of those teams you mentioned have numerous challenges that would not allow them to compete with MLS.
1. Indy Eleven- Stadium is not suitable, and attempts to get a purpose built stadium so far have failed.
2. San Francisco Deltas- Stadium is hardly a step above Hofstra and this franchise has no track record yet. Getting a 20k+ stadium built in SF is harder than NYC.
3. FC Cincinnati- Lot's of promise, but only one season so far.
4. St. Louis SC- Not really lighting the world on fire, doing ok.
5. Tampa Bay Rowdies- Nice club with aggressive owner, nice stadium but nowhere near MLS level.
6. Jacksonville Armada- Not a good stadium and attendance cratered in season 2. Owner appears incompetent.
7. Arizona United (Now Phoenix Rising SC, terrible name!)- No track record, no existing MLS level owners.
8. Pittsburgh Riverhounds- Lol.
9. Sacramento Republic- Great club with MLS ready stadium plan!
10. Ottawa Fury- Decent attendance, but 2nd rate market.
11. Louisville City- Not capable of competing with MLS clubs.
12. Carolina Railhawks- Maybe. Decent attendance.
13. Miami FC- Not sure what the hell this club's vision is. Tons of money, would have same stadium struggles as Beckham.
14. FC Edmonton- Lol
15. Nashville SC- Not very familiar.
16. Oklahoma City Energy- They draw a couple thousand out to a high school football stadium.
17. New York Cosmos- Would need legit owners and an actual stadium plan.
18. San Antonio FC- Sure.
Basically, these are mostly 2nd and 3rd rate markets, which would kill them on TV revenue. Mostly unsuitable stadiums, and mostly unsuitable owners. The cost of entering MLS is most likely significantly lower than the investment it would take to bring a rival league to the same level as MLS. Most of the above clubs who could actually be a viable 1st division club would more likely choose MLS instead of spending a ton of money on to form a rival league with much higher risk.