A Monster Manual II is definitely one of the purchases I am waiting to make. I may only use a tiny percentage of the creatures, but I am a lore junkie, and love reading up on the creatures.
The 5e Monster Manual has done a fantastic job of inspiring me to use monsters that classically I would have never used. So, the concept of a second book which keeps that trend going is exciting to me.
My 5 monsters would be as follows:
Lesser and Greater Elementals - I know it is an uninspired desire to have these bad boys added to the repository of monsters, as they are easy to build yourself. However, I would love to have the option to more easily make elementals into minions, mooks, and serious threats across a wider level range. Hopefully with this addition to the monster manual we would also get para-, quasi-, and various other odd elementals added to our repository.
Inevitables - I love these guys. I love them a lot, and would love to see them in 5th.
Sidhe - We need more fey, and more specifically I need more fey. Having some upper level fey threats that really make the party think twice before entering the woods is the stuff that storytelling is made of.
Atropal - Man.... have you seen this thing. Why would a DM use this? I mean seriously. Yikes. I would love to answer that myself in my 5e game.
Boggles/ and Banderhobbs - Banderhobbs is my number one monster I want, but OP already claimed them. So, I tacked on Boggles as well. These are the two monsters that made my players fall in love with my 4e campaign and I would love to see how 5e would tweek their lore, and abilities.