Three down, one to go

Is this an Arena core team? If so, then you don't really need a fourth merge project since the fourth member would be a bonus unit.

But anyway, I'd probably go with a mage. Lukas is a physical wall that deals physical damage. Fae is more of a magical wall that deals adaptive damage, and Marth is kinda like a physical wall that can buff and heal.

Or you can have a dagger unit. Maybe someone like Felicia, or Saizo, or Kaze. Or a healer like Absorb + CC Lachesis, or mage tank Wrys.

If you plan to go into high tier Arena (20-21), you gonna be fine with Fae. Marth and Lukas doesnt have the stat to maintain their position in arena these day because of how competitive it is. For general use, FTP mages like Nino, or Lilina, Reinhardt (for content clearing not arena) is good for investment.

First of all, grats on completing your Lukas! Looks like you could use a ranged unit for sure. A few good merge projects that could benefit from your Infantry Pulse setup are Sothe, Soren, Leon, Lachesis, and Lilina.