I think Ancelotti understand the importance of timing and have seen that it is better to leave when he is still seen as a hero rather than when things have gone badly. He has made Real Madrid win trophy for most of the season and going a season without trophy might make him lose some of those admiration he enjoyed. This is the reason he might be considering leaving to ensure he do not spoil his reputation. I completely understand him and I will support whatever decision he takes.
Ancelloti has won almost every major trophy during his time as a coach and maybe a season without a trophy is not a big deal for his reputation. La Liga still has a chance that they can chase as long as they can make a much better performance because when they face El Clasico they can close the gap on Barcelona even though they will still be one point behind Barcelona.
Barcelona will be more focused on preparing for the semi-finals in the Champions League so the coach will definitely make some rotations and if the results are beyond expectations because they will draw in the next match then this scenario can be used well by Real Madrid so they can chase points in the crucial match when the two meet.
Hamsi Flick is an experienced Coach and he has won the Champions League before and he has won the treble during his time at Bayern Munich, So at this point I know he will have his eye at Winning the treble, I don't think resting players should be an option for him as a manager, he is well experienced to manage his side with the pressure attached to it as well, And the Champions League game will be in two weeks from now so I think there is a lot of time to prepare his teams now, If I was Hamsi Flick i will not rest players mean while I will use all my players for the game against Celta Vigo because Celta is a very big team, and in recent years they have been a team that have been a tough opponent to Barcelona.
I feel this will be a very tough game for Barcelona, But Barcelona at home I will predict Barca to Win.