Every club in the Saudi League has strengthened their teams in such a way that it is difficult to distinguish between the big teams and the smaller teams. Ronaldo and Nasser play for the club and some other star players have joined the Al Nasser club so we consider this club as a strong club but outside of that there are clubs like Al Hilal and Etihad who have been performing very well. No match for Al Nasser is easy, every match requires a lot of hard work and proper planning to win. One season Al Nasser is the champion, another season Al Ittihad is the champion and another season Al Hilal is the champion, so there is still competition in this league and that is definitely a good thing.
It's easy, in order to know which are the big teams and which are the smaller teams depend on how much money they have, if they have a lot money, they're big teams and they able to sign European players. There's a competition in this league, but a competition among B class teams. 
If there's an A class team joined the league, they can easily win the league.
I think that currently almost all SPL team have developed very well and almost all team have their mainstay players who are players who previously came from Europe, in the next season or two it is very likely that the SPL will become even more popular.
They are busy trying to bring in European players, several names in every transfer window that is opened always receive attractive offers from several team, in the future these players will definitely come to increase competition in the SPL.
Al Hilal, Al Nassr, Al Ittihad, Al Ahli and also Al Taawon are the dominant teams which are quite competitive in the competition for the title even though there are also several other teams which previously were able to perform optimally like Al Shabab, but unfortunately they performed with decline in the season.
If in the future the competition becomes very fierce then getting the title will not be an easy matter because they will maintain each other stable performance which can be used as determinant in winning the title.