Yes, it could happen that Lionel Messi will break the record set by Cristiano Ronaldo while he was playing in the Champions League. 12 goals is the goal difference between these two players, so that's a very likely thing to happen, especially now that Cristiano Ronaldo will not add to his coffers because he is no longer in the Champions League, while Lionel Messi will continue to be active there. Lionel Messi's contract extension will also increase his chances of returning to play in the Champions League next season with Paris Saint-Germain. Because I see that it is certain that Paris Saint-Germain will return to participate in the highest football caste in Europe.
It will be difficult for Messi I think if he is not going to play more than 2-3 seasons.In Champions League Messi has not scored with Paris as many goals as he has scored with Barcelona and I doubt this is going to change.Paris has its main objective to win the Champions League and they will not care much of furnishing Messi with a lot of balls,they just need to get past their adversaries and score.
The time though clearly permit to Messi to score more than 12 goals during these 3 seasons which I am assuming he will be playing including this one we are already in the middle of it.This does not mean that if Messi scores more goals than Ronaldo is a better player than him,they both have won a couple of times the Ballon D'Or so for me they are the two greatest players of our time,as simple as that.
But it really depends on Messi too, we don't know what the future will be like. maybe this season he will still stay at PSG and the guarantee of the Champions League is still there but we won't know what will happen in the following seasons.
It's only a matter of time whether we can answer that or not.
As for the greatest players at the moment, I think the word GOAT for them is enough to prove that they deserve to wear that as their current nickname.