Metal-O for Metallo does sound lame to me. Sorry , just does. That said ,I think the script is fake, so I'm not gonna get into a debate about elements of a fake script or whether the source of those elements work on their own merits.
As for the character himself being in MOS 2, it makes more sense for this Batman/Superman film then does Doomsday imo. I think the character could work as a secondary heavy, though I could see then going with Doomsday because a threat like that may justify have Superman, Batman, WW and who ever else turns up having to unite to stop it.