The first and second ones were the MCs with proper monsters, and honestly, a lot of them don't need to be converted, IMO. Some of them are redundant: There's already animated armor and helmed horrors; they don't need doomguard as well; and assassin imps are just imps, really; and do you really need both alven and pixies, or reavers and sahuagin? Animators are basically undead living animate objects. There's also "greater" or more powerful "Ravenloft" monsters, which aren't necessary now that it's easier to increase the power of a monster , and hopefully they'll give us a bunch of Van Richten-style ways to modify monsters with special unique traits.
Some of the undead in there are basically other undead but with a slight twist or in a different environment (like bowlyn). Other monsters are more like effects (bussengeist, those viruses) or magic items (wishing imps) than actual monsters. Some are very domain specific: if they don't do Forlorn (can't remember if they are), would they do goblyns?
And I doubt they're going to do Darklings again.
I mean, there's totally still a ton of RL monsters that deserve to be converted, but I'm not sure there's an entire MM's worth, considering they haven't put out a generic MMII yet.