I think for an April Fool's joke we should go for either extreme: one that's believable enough to make fools of anyone who thinks it's a real game, or one that's not quite believable but is at least amusing/fun/interesting.
We all know why I wanted to mix Mega Man X and Metal Gear Solid (interesting boss concepts, blah blah blah). I've always thought Mega Man Solid X: Guns Of The Mavericks would have worked better if Mega Man (X) fans and Metal Gear (Solid) fans were the same people. Most people I know are only one or the other but not both. Perhaps, if this site were a more general video games web site, and not one about maps, we could've presented bustin98's artwork alone as supposed concept art and the point wouldn't've been missed. I remember him saying his friend who worked at a games store printed off the artwork and fooled some people there.
"Metroidvania" and/or "Castleroid" have been terms used for the genre of games, I'm trying to envision what a game that was a mix of the series would look like. It truly isn't believable; at least with MMSX:GotM you could see each map to see bustin98's rendition of another character, whereas I don't know that we'd necessarily want to render a bunch of areas in Metroidvania when the "joke" is already over. Unless you want artwork of crossed bosses like "Death-Ridley" but then the maps become secondary again (and besides, there aren't many other memorable bosses).
On a not-quite-related note, a steampunk version of Samus's suit would be interesting...