From what I can see the Dutch/Belgian league is replacing their national leagues and is viewed by Uefa as a domestic league. The winners of it will then go into the Champion's League, runners up into Europa and so on. I don't know how promotion and relegation work. There are currently talks between the Irish League and the League Of Ireland over a combined North and Republic domestic league now too.
In any combined league it probably won't be difficult to point out the clubs who'll be more successful. Ajax, Feyonoord and Anderlecht are simply bigger clubs from bigger cities than Ghent, Willem or Alkmaar and enjoy the advantages that brings. In theory, with equal resources, you'd expect Rangers to be competitive in any domestic league after a couple of seasons. Our support and club structure and culture should allow us to be competitive. Famous last words of course.
Personally I think combined domestic leagues is inevitable, it's just a matter of how long it'll be before they happen. The only other question is which version we join. I would prefer if the Scottish league could improve and we didn't move too, but it just seems the momentum isn't in that direction. That said if the money bubble in football bursts, and while it's unlikely it does seem like a ceiling might have been reached, then maybe there's a big reset coming for all clubs.