Originally the plan was for Marcel to help destroy the wall then he and Annie to join up and go into the Military Police together.

Even though Marcel died they(Reiner, Bertholt, Annie, and Zeke)continued on with the plan knowing they already made it too far to stop then.

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I feel like this is what would've happened if Marcel didn't die.

We have no clue what Marcel's Titan powers were but if he was apart of the mission then he would have to be strong.

I believe that if Marcel lived then the plan to destroy all the walls and kill mankind would've worked.