As luck would have it, TheFatPanda found one more piece of the Worlds Unite cover mosaic, this one for Mega Man #51, whose solicitation you can see otherwise in full here.
For the cover, here it is by itself:
And here it is as a part of the collective, completing the second of three rows:
Hmm, that doesn't look quite right, does it? Let's adjust it a slight...
There, that's more like it! ...sort of. Hopefully the version that goes to print will line up properly and won't leave that unfortunate gap there.