I was originally thinking something like regenerate, but I realised this could fit better.

It's almost never printed, I guess to avoid reanimation shenanigans. Or did I just screw up my search? But in a precon deck there won't be any Emrakuls to return.

In many ways it would fit the flavour better if it only worked on a newly-dead creature. Maybe I should revert to regenerate. Except that ideally it wouldn't save it permanently? Something like, "Regenerate target creature. It gets +2/+0. Sacrifice it and the next-but-one end-step" but that's obviously v fiddly.

"Return target creature that shares a name with a creature that died this turn from the graveyard to the battlefield. It gains Haste." ?

Simplified Goryo's Vengeance. I don't see a problem with removing the Legendary rider and Splice onto Arcane at the same time. The Vengeance seems to infer that this could cost ? Don't need to, though. Three mana seems fine. Is the exile part important?

Ah yes, I forgot Goryo's Vengeance. I agree with the cost comments.

I'm guessing exile is a safety precaution against an infinite loop of a biggish creature that recurs an instant. I think it would usually be prudent but I'm designing a specific precon deck so I think I won't bother, I'll have the simpler version.

Returning the creature the turn it died makes sense, but I don't really like it as usually creatures die during combat in which case it's useful.