That was a joke, Twilight. It was a joke. I don't think the bloodline destined to slay Grima can host Grima. :p

I'm pretty sure it was discussed somewhere down the line that MaleMU become Plegia's king, marries Aversa, makes baby, generations generations, Laius pops out. 

So if you really want to bring back Grima, just say one of Laius' children went all dark mage and sacrificed a few virgins or something.

But you did say that Grima was killed in this timeline. Which means he can't come back, ever. :p

I guess if you pulled some time travel shenanigans you could reveal the future Grima. Say he completely torched the future timeline without Lucina there to stop him, so now he hungers for more worlds to devour.