Hello, welcome to EDH!

ADDS:

Firstly, 27 lands is not enough, especially in a three color deck. I would aim for 35-37 in this deck since you have 6 mana rocks.

If you are up for ordering cards, Command Tower should go in any commander deck with more than one color.

Evolving Wilds is exactly Terramorphic Expanse, but having both doesn't hurt for consistency.

Recently rotated from standard lands that are pretty cheap right now: Sunken Hollow, Hissing Quagmire, Lumbering Falls

Pain lands are pretty cheap too: Yavimaya Coast, Underground River, Llanowar Wastes

Counterspell is strictly better than Cancel.

You are going to want Swiftfoot Boots to protect your commander and give it haste.

There are 2 different ways you can go about building the deck.

1. You can go for easy wins with Blighted Agent and Jodah's Avenger.

2. Or you can go for more fun stuff like Mycoloth, Sage of Hours, Master Biomancer, and Cold-Eyed Selkie.

Either way is valid and enjoyable, it just depends on who you play with and how you like to play.

For either one though, you need more big creatures for Mimeoplasm's second targer for counters. Some good cheap options are Krosan Cloudscraper and Ghoultree. Inkwell Leviathan and Gaea's Revenge can be used as targets for the first or second ability.

Terastodon can be used as removal and then as counters later in the Mime.

REMOVALS:

Firstly, Gifts Ungiven is banned in commander. If you are playing casually it shouldn't be too big a deal, but I thought I'd let you know.

Artisan's Sorrow or Relic Crush. 3/4 Enchantment/Artifact Removal is a little overkill.

Metallic Rebuke is not very good here because the only non-creature artifacts you have produce mana, so you can't really use the improvise that often.

I don't know if you are going for a recursion theme, but generally in this deck you want your creatures to stay in the graveyard, so Wander in Death, Fortuitous Find, and March of the Drowned don't seem very good in this deck. A little bit of recursion is good in any deck, but this is too much.

Hello, and welcome!

I'm in class currently so I can't look too thoroughly at your list, but I can post mine: The Mimeoplasm V3 for you to compare and get some ideas from. Mine is a bit more of a combo list but there are some very good cards that are relatively cheap and very effective.

From what I can see, I would add more effects that bring cards from your graveyard to the battlefield as your main form of recursion. With mimeoplasm your graveyard is one of your most important resources. You want the ability to be able to get whatever you want out of it when you need it. I would also include a little more point removal and maybe some more ramp. You can never go wrong with a Beast Within or Explosive Vegetation.

I would also add some things to put cards into your graveyard. Deadbridge Chant is an excellent way to hit multiple of these points and is very cheap as well. It is fairly efficient to get the engine running in my deck and can sometimes slip under the radar until you bring back something huge.

The Mimeoplasm is an absolute blast to play, and there are so many ways to play him. My best advice is to decide on what you want your wincon to be, and then attempt to get to your wincon faster and faster. Whether that's comboing out with counter strategies like Sage of Hours / Triskelion , or with commander damage using Death's Shadow and Gaea's Revenge to one shot people, or even milling your oppenents out or milling yourself to win with Laboratory Maniac.