Octopus Umbra, Howl from Beyond, one or two of the "shapeshifter" creatures that counts as all creature types

More ramp? Cultivator's Caravan is obviously absent, maybe Aerial Surveyor.

Unlicensed Hearse is amazing vs decks that use the graveyard.

Some of the creatures with "Inspired" produce tokens as they untap, that might fit your theme

I'd trade out maybe 5 lands for 5 land tutoring sorceries if you want lands faster. Skyshroud Claim, Rampant Growth, Nissa's Pilgrimage, Cultivate, Kodama's Reach etc.

Since most creatures will be tapped, I run Virtus the Veiled and cards like that with my Thantis deck.

I have had a lot of fun using creatures with Dash as the base for Mutate. At the end of the turn the Dasher and the Mutates return to your hand so you can use the effects over and over. Also, Cephalid Constable with Mutate can be really effective.

Being legendary only means you can only have one in your deck in regular magic and in commander you can make it a commander. But you are absolutely allowed to put it in your deck

Oh, and just for fun you need to put in at least one creature with "Annihilator"

Clamor Shaman, Goblin Heelcutter, Krenko, Tin Street Kingpin, anything with "Mentor", Bishop of Binding and Keleth, Sunmane Familiar are all cards that would work well with your commander. The Legendary Enchantment backgrounds from the latest set (Baldur's Gate) are cheap and have some great commander based bonuses.

Master Chef and Raised by Giants from the new commander set. A bonus counter everytime Skullbriar hits the battlefield and a base power boost equivalent +9 counters

Presence of Gond, Paradise Mantle, a Vehicle or Predatory Urge would let you tap at will

If your commander is a Planeswalker you are going to need a way to protect it. I think you need a few more creatures, probably more with trample or at least bigger than what your opponents usually play. Deus of Calamity, Primeval Protector

If a creature is equipped with something that gives them Trample or Haste (like Chariot of Victory or Sword of Vengeance) you can target them with Misc's +1 ability.

Red spells like Act of Treason would allow you to steal a creature, attack and then sacrifice the creature with Minsc's second ability.

Longshot Squad and Tuskguard Captain both give bonuses to creature with all those +1 counters Minsc can throw around

I would trade out 3 or 4 lands for something else. Your deck runs cheap on mana.

Some of the creatures with "Outlast" give bonuses to all creatures with counters.

To take advantage of the "deals damage equal to its power" maybe you should throw in something like Blackblade Reforged or Lunar Frenzy? To take out creatures with Indestructible you could add Fists of the Demigod or maybe some Infect for the -1 counters.

Cephalid Constable (a little pricey), Tanazir Quandrix and Cold-Eyed Selkie benefit from counters

Acidic Slime, Herald of Secret Streams, Iridescent Hornbeetle, Managorger Hydra, Sakura-Tribe Elder and Trygon Predator are all useful and have creature types I did not see above.

Literally did this same thing for my son. Dovescape freaked out so many people we built a deck around it. Our deck is titled "Have you heard the word?" if you want to look at it (not quite as budget as your deck, around $175).

You need to mark your commander by typing CMDR after it, flanked by asterix (if i type it out here it just adds italics) (we used Ishai, Ojutai Dragonspeaker)

Aven Mindcensor is awesome. It even stops ramp when people realize Cultivate might not even net them a single land.

Thieving Magpie and Spear of Heliod are both very useful.

Battlemage's Bracers could double your goblin production and still fit your budget build. I also like using Cosmotronic Wave with goblins.

Wave of Reckoning, Loxodon Wayfarer, Rhox Faithmender, Rhox Meditant, Rune-Tail, Kitsune Ascendant  ^Flip, Serene Master, Commander Eesha, God-Eternal Oketra, Karametra, God of Harvests, Oketra the True, Spirit Loop, Holy Armor, Consulate Dreadnought, Zetalpa, Primal Dawn, Grizzled Leotau, Sigarda, Heron's Grace, Trostani, Selesnya's Voice, Dragonlord Dromoka, Hand of Justice, Myojin of Blooming Dawn, Radiant Solar, Kami of Old Stone,