Starting from the era of Ronaldo and Messi the La Liga has never been as competitive as the English Premier League and neither has there been plenty clubs competing for the title. It has always been Real Madrid and Barcelona and sometimes Atletico Madrid and the likes of Sevilla who will also will be at 5th or 6th position.
Ronaldo appeared in Real Madrid for training, not with their squad but he trained with his personal coach. No confirmation that Ronaldo will join Real Madrid for rest of season and his chance is slim. Beyond reasons related to his poor form recent months, a recovery of Benzema is another reason Real Madrid might not need Ronaldo.
I have never seen a big upset where an underdog takes them by surprise just like what happened in England with Liecester City.
In La Liga, dominations of Real Madrid and Barcelona are too big and in the past season, we have Barcelona with best ever time in their club history and Real Madrid that managed to won 5 Champions League within 10 years. Considering it and thinking about strength of Barcelona and Real Madrid, how other clubs can win La Liga.
In the past, there were some under dogs like Valencia, Deportivo La Coruna (anyone still remembered this club), Atletico Madrid a few years ago and I believe in coming two to three years, other clubs might have great chances to beat these giants. Real Madrid must reshuffle their teams with some legendary players will leave or retire. Barcelona still need more time for their young players to improve ability to carry pressure of winning titles.