I seriously doubt they would let them play at UCL, not a chance. I understand that these teams are spending a lot of money, and they are getting a lot of views as well, but we are talking about getting in early, and even if the yare "allowed" some way, it looks like their league is way below to be considered, they wouldn't get all that much points.
Remember, national league points are the ones that decides which teams gets to send who to where, and bigger nations send 4 teams to UCL, whereas smaller leagues not only send just one, but those ones end up playing knock out stages too.
This means that we are going to end up with a situation where even if they are allowed (which I doubt) they would still not be able to send much.
Speculation arose because Ronaldo was in the Saudi Pro League, not because they saw any improvement in quality in this league, so it is clear that they are only trying to increase the number of spectators at UCL matches because of Ronaldo's presence.
If that happens, maybe there will be leagues in Europe whose participation numbers will be sacrificed, but the problem is how much SPL will get a share? and also if the target is only Ronaldo, that means Al Nassr must be in the UCL spot in their league standings, what if the club also fails?
But what concerns me more is how they can compete with teams from Europe, I doubt there will be anything surprising that clubs from the SPL can do in the UCL. I think something embarrassing will happen where they won't be able to win even once in the group phase, and they also have to think about changing the name, no longer the UEFA Champions League because the Saudi Pro League is not included in UEFA.
The most likely thing to happen is that FIFA will create another competition to make this happen, where European teams and the SPL compete in one competition, but another doubt arises whether the number of spectators will be like the number of spectators at UCL? because we know now that football is not just about sport but business..