It's no secret that Real Madrid is very interested in signing Mbappe, so if the contract really will be realized, I think Mbappe will be the first choice along with Benzema. With Mbappe's current form and the high salary he receives, it would be strange if Real Madrid chose to leave Mbappe on the bench. Even the most likely formation is to play Mbappe, Benzema, and Vinicius simultaneously (I think this trio will be a scary threat for Real Madrid's opponents).
Mbappe didn't started his career as a striker, he started as a winger at first, then eventually moved towards striker as he aged, just like how Ronaldo became a striker for the past 4-5 years for example. This could be the same situation. Maybe they will do Junior-Benzema-Mbappe trio at the front, which would be a great situation for them.
This could be a bit of a difficulty to arrange since Mbappe would prefer to be at left if he is going to be a winger, but even at right he would be great. Or at the worst case scenario, they will simply play with two strikers and that would be enough. There are tons of tactics with two striker formations and it could work well.