So apparently we're getting Momir in arena. I'd never heard of it, so I googled it. I suggest you do the same, or just check out this link:
https://www.mtggoldfish.com/articles/in ... omir-basic

So it's all about cc of creatures. There aren't any landwalk abilities in our cardpool, so that's a major factor vs traditional momir, so basic land choices for your deck will be different too.
So what creatures do we have in each cc slot that are relevant?
Let's start at the top:

12 cc (1)
Ghalta, Primal Hunger
At 12 mana, you have a guaranteed ghalta, which is big and nasty.

11cc (1)
Metalwork Colossus
At 11 mana you have a guaranteed colossus, which is big, but very chumpable.

10cc (0)
So 10cc is a dry patch, so if your going for ghalta, you'll have to skip this CC.

9cc (1)
Zacama, Primal Calamity
At 9 zacama looks pretty damn strong, but it is legendary, so no use spamming the 9ccs.

8cc (9) (At 8 things start getting interesting)
Wakening Sun's Avatar (not cast so ability is irrelevant)
Zetalpa, Primal Dawn (so many keywords...)
Slinn Voda, the Rising Deep (can't kick it, so another irrelevant ability)
Razaketh, the Foulblooded (flying, trample, pay 2 life, sac a creature: search for a land?)
Torgaar, Famine Incarnate (life-gain/lifeloss could be relevant)
Ancient Brontodon (vanilla 9/9)
Polyraptor (could get interesting)
Verdant Force (A great source of chumpers)
Gishath, Sun's Avatar (hasty fatty, but sadly, there are no dinosaurs in your deck)
Zetalpa, verdant force and polyraptor may be the best creatures for the 8 slot.

7cc (32) (lots of nice stuff, but some chaff too)
Exquisite Archangel
Goring Ceratops
Seraph of the Suns
Aethersquall Ancient
Cryptic Serpent
Fleet Swallower
Gearseeker Serpent
Nezahal, Primal Tide
Striped Riverwinder
Windreader Sphinx
Fen Hauler
Herald of Anguish
Kazarov, Sengir Pureblood
Angrath's Marauders
Chaos Maw
Silverclad Ferocidons
Aetherwind Basker
Garruk's Horde (irrelevant ability)
Ghastbark Twins (not too shabby)
Greater Sandwurm (boring)
Pelakka Wurm (decent)
Sifter Wurm (not much to sift for)
Thorn Elemental (this is a good finisher)
Thundering Spineback (okish...)
Verdant Sun's Avatar (this looks annoying)
(who knows, it could work out...)
Tishana, Voice of Thunder (might work if you lots of creatures)
Chromium, the Mutable (looks ok even if some of the abilities like flash are irrelevant)
Darigaaz Reincarnated (oh yes!)
Accomplished Automaton (bad)
Barricade Breaker (pretty bad)
Meteor Golem (small, but could be useful)

I'll finish later...

Consider the kicker to be integrated and you could put Slinn Voda, the Rising Deep in the 10 cmc slot. I haven't seen Merfolk decks that much any more and consider the rarity of running most of the other types I'd say the ability is pretty nice for the kicker.

No spells are cast in Momir though, so Slinn Voda's kicker ability can't be used at all.

Hard to imagine how this won't just end up as a Zacama fest. It's going to be the default line on the play, meaning that on the draw you'll likely need to kill your Opp before they untap with one on T10. Making a Zacama of your own after an Opp has one does nothing, they can just mow yours down when they untap.

I'll give it a try, but I'm not super optimistic about how fun it'll be at the moment. Maybe in the future with a more robust "Arena Modern" pool to pull from.

.....ok
I obviously have no idea what Momir is other than the ex Simic Guildmaster.

I don't see the appeal. If I wanted RNG on top of RNG on top of RNG with some RNG in between, I'd play Hearthstone.

Do they want to lure HS-players with this mode?

Just giving you more stuff to do should you get bored. They said one of their guys implemented this in his free time, so it was free for them.

I've never tried it, so I guess it's worth a spin, even if just to try it out and take a break from regular magic.

The RNG is real... No skill necessary, just drop the most busted creatures and hope your luck doesn't run out. Then, whoever gets to zacama first with 3 red mana controls the board (beware of auto-tap) and wins.