I thought this would be an ok idea but I don't know if these teams would be strong enough.
Although, they would probably start gaining more income as I would imagine premiership teams earn more with the TV rights etc and this may allow them to buy stronger players.
A major disadvantage would be that Rangers/Celtic can gain champions league football in the scottish league but if they come to the premiership and don't finish in the top four (which they won't) they can't get in champions league.
I had another idea that maybe the top four finishers in the scottish league could be entered into our FA cup or something, we would still get competative edge between the clubs/supporters and the scottish clubs wouldn't loose their european spots....