I feel like Solidarity of Heroes would be a solid addition.

Sage of Hours is another one to consider. It seems like it might be difficult to get off, but it's not like you don't have stuff that would stack up some counters.

And maybe Mikaeus, the Unhallowed , He'll give your creatures recursion and let you reuse some ETB effects, not to mention he goes infinite with Triskelion

Mikaeus, the Unhallowed is an amazing card, however, all of my creatures are going to have counters on them so undying won't work unless I get more cards that allow me to move counters.

That's a fair point, have you considered any creatures with the Graft mechanic?

I just looked up some cards and found Cytoplast Manipulator to be powerful for its ability to gain control of other creatures.

Decree of Savagery would have a great home in this deck.

Seems a little expensive, or is that just me?

in commander you should have the mana to cast it unless someone is playing land destruction.

This is what I was looking for in a commander deck. I have some really good suggestions.

*Lotleth Troll - He has too many plusses. Low cost, trample, booster, with regenerate. Combo him with Nasty Jack and/or Kalonian Hydra and he is a beefy beatstick that they can't get rid of too easily.

*Skullbriar, the Walking Grave - He is amazing as a commander, but still great as a non-commander monster. If he ever leaves the field, you can exile him with Varolz for cheap.

Korozda Guildmage - Combos with a lot of creatures, including Necropolis Regent , Skullbriar, the Walking Grave , and Vorosh, the Hunter . While the chances of you being blocked are high, since you have 3 colors, you can also use the second effect. You should have sufficient counters to be able to swarm the field with saprolings.

Black Oak of Odunos , *Springleaf Drum - Combo with Gilder Bairn . there are other cards like this. Gilder Bairn doesn't have to attack to untap now.

Jarad, Golgari Lich Lord - He is especially great in multiplayer.

*Plaxcaster Frogling - Great cheap protection

Simic Manipulator - because stealing is fun, and totally fair.

*Llanowar Reborn - Helps boost one creature. If Vorel of the Hull Clade is out, you can boost it to more counters. You can also bounce it back to your hand with Golgari RotFarm or the equivalents.

Golgari RotFarm, Dimir Aqueduct , *Simic Growth Chamber - They are too good at mana fixing.

Breeding Pool , Overgrown Tomb , *Watery Grave - Also really good mana fixes. A bit expensive, but they help if you can get them.

*Vigean Hydropon - Great graft card.

*Cytoplast Manipulator - someone already recommended her. Again, stealing is fun, and completely fair.

*Kiora's Follower - Repeat the effects of; Gyre Sage , Cytoplast Manipulator , Vorel of the Hull Clade , Astral Cornucopia , Bow of Nylea , Doubling Cube , all your other artifacts, and your lands.

Sorry for the tl;dr. Long list. I put asterisks next to the higher priority stuff. I am pretty sure I had more on my mind. Black has killspells. You need killspells. Also, a note on graft. Corpsejack Menace has no effect on graft, since the monsters enter the field with the counter.

Cyclonic Rift is a good instant one-sided boardwipe, Sol Ring is fantastic mana ramp, Lay of the Land seems kinda weak, maybe replace it with Kodama's Reach or Cultivate and if you use Farseek you should get some ravnica duals to take better advantage of it. Decree of Savagery could work and Alchemist's Refuge or Leyline of Anticipation to play cards on opponents turn.

Also Scute Mob because its cute as hell :)

I would just like to add, all the lands I added are suggested as important. For some reason, the asterisks didn't show.