Here are some great and usually big monsters in EDH, some of which might be too powerful for your playgroup, so use your discretion.
[[Lord of the Void]] is a fun one in EDH as there will always be good creatures to steal. [[Sepulchral Primordial]] is amazing too, as it reanimates something from each opponent. By that same reasoning, [[Diluvian Primordial]] is also pretty good ([[Sylvan Primordial]] is also really strong but is banned).
[[Rune-Scarred Demon]] is a staple in black, and you can tutor it out with green spells like [[Tooth and Nail]] or [[Chord of Calling]] too.
[[Sheoldred, Whispering One]] is probably essential to a Mimeoplasm deck as you will have creatures in the graveyard to reanimate anyway, and she also gives swampwalk to your Mimeoplasm when you target her. [[Vorinclex, Voice of Hunger]] both ramps you and hampers your opponents. [[Jin-Gitaxias, Core Augur]] does the same for card draw, and if you can keep him alive you probably win. [[Consecrated Sphinx]] also gives you plenty of card draw, should help you fill your graveyard with the right creatures when you discard as well, and of course gives evasion on a decent-sized body when reanimated with Mimeoplasm. [[Damia, Sage of Stone]] draws lots of cards too, is in the right colors for you, and has deathtouch. [[Jarad, Golgari Lich Lord]] can also get very big with your strategy, and [[Glissa the Traitor]] also has first strike and deathtouch. I presume you're already running [[Sidisi, Brood Tyrant]], although she isn't that big.
[[Massacre Wurm]] acts as a bit of a sweeper, and can destroy token players. [[Kederekt Leviathan]] can be a very powerful card to bring back with Mimeoplasm, especially if you have a [[Concordant Crossroads]]. [[Terastodon]] is great, whether to destroy other problematic permanents or just to sacrifice your stuff for some extra tokens.
Copy effects are especially great, so try things like [[Clever Impersonator]], [[Phyrexian Metamorph]], [[Body Double]], and [[Sakashima the Impostor]]. [[Duplicant]] is a similar sort of card that also exiles their creature.
[[Phyrexian Obliterator]] and plenty of counters on a Mimeoplasm is practically an Eldrazi. Speaking of Eldrazi, [[It That Betrays]] is great if you can ramp to it, since sacrificing is a big thing in EDH.
[[Avenger of Zendikar]] is generally good in all green decks. [[Siege Behemoth]] is an underrated way of always getting your damage through, and it is also hexproof.
For some protection, you could also go with [[Thrun, the Last Troll]], [[True-Name Nemesis]], or [[Gaea's Revenge]].
Hope that helps! Now I feel like playing a Mimeoplasm game...