Umber Hulk
Mimic
Chimera
Basilisk (since the medusa is coming)
Shambling Mound
Elder Brains (Since we already have mind flayers)
Kenku (come on, a bird using a staff? awesome!)
Gorgons (reminds me how about Demogorgon)
Xorn
Prismatic Dragon
Shrieker would be annoying - it would just sound like a pug of prepubescents playing the game. I guess you could get Tarrant to voice it (ya know, that noise he makes when he gets in Monday morning and starts perusing the forums).
What would be cool is some sharks in the harbor that jump up and eat people on the dock (like Samuel L. Jackson in Deep Blue Sea).