Aside from the ones you mentioned, I think that there is probably room for a Limerick team too - Limerick Junior football is jumping with good well organised clubs. Whether there would be interest in moving up a grade is a different matter, especially seeing the grief that the extant side face on a weekly basis. The big mystery to me is that there is still no team from Mullingar in it, especially given that two different ones both applied to join the league proper a number of years ago.
Tipp could feasibly host two. Thurles Town used to play in the league in the 70s and 80s for a short spell, while Clonmel has always been a good team. Indeed, in the 1920s when they were initially talking of a second division, Clonmel, Cork and Waterford were seen as places with potential for expansion. And if Galway can have three clubs in the National Leagues, there should be room for another side or two the A league for Cork. Obviously Dublin may provide easily another side or two.
The big question is whether it is worth the while of the big fish in small pools doing the whole change of season and then trying to swim in the strong currents. I can see a lot of very good Intermediate and Junior clubs not even considering it.