Lemar is a playmaking attacking midfielder. Like Mahrez, he would be a wide playmaker and another source of creativity, not a wide forward who would replace Alexis' goal scoring. In the 4-2-3-1 he would be balanced by a wide forward like Alexis or Walcott on the other side. In 3-4-3 it is trickier - he could play behind the striker with Ozil but we'd only have one real goal scorer on the pitch. But he could also play as left wingback in certain matches and situations.
I could see Wenger molding him into the Cazorla deeper lying playmaker role but only over time.