Some scattered thoughts...
Maybe both Golos and a companion can be used, in order to have multiple mana sink options? Golos might want to pull in black mana sources (black lands, Deathrite Shaman), which then opens the door to tutors, which in turn could bring more consistency to a highlander deck. Not to mention the Coalition Victory dream...
Alternatively, if going down the Yorian route, perhaps the Risen Reef/Young Pyromancer/Chandra, Acolyte of Flame package could make sense. After all, both Omnath and Yorian’s ETB have some synergy with Risen Reef. Your RUG Elementals deck thread noted that the inability to run Force of Vigor due to lack of green cards hurt the deck; maybe the addition of Omnath and Sylvan Library can help reach the required critical mass of green cards. Also, I think the cost of having a card like Blightsteel in your deck diminishes the larger your deck size, as the probability of drawing it decreases, so Yorian might justify bringing in a Tinker package...which might warrant bringing in Golos as a potential Tinker target.
Volrathxp appears to have tried a Yorian Elementals deck, using an inferior Omnath.
Congrats on the finish.
I think you are definitely into something in terms of a mana sink. I’m not sure if a Companion is the best option there, they are nice for the guarantee, but the deck limitations are pretty severe for the effect now with the tax included in their cost.
What other mana sinks are there? Well there is Urza, and there is also Golos. That is a couple of very powerful ones, especially with big spells in your deck.
You can also just go down the route of using him to reliably generate 4 different colors of mana. This may be the intersection with a companion, Jegantga, but that would exclude basically all pitch spells. So the deck would need other ways to interact. You can throw in a Coalition Victory here as an out to instant win some games.
This is to say there are probably a few routes one could go with this.
If you are running a companion, you always have something to do with your Omnath mana and you don't have to run cards like Ugin that might be dead draws if you can't stick and Omnath.
As for particular tech, Sevinne's Reclamation is great, as a value card, as a mana sink for Omnath mana, and as a way to hit multiple land drops in a turn if you need to (used it against Dredge to get back Tabernacle).
Had several powerful sequences enabled by Omnath such asCompanion Tax on Lutri, Ancestral, Lutri Omnath, Draw Narset, fetch, into play Narset, reveal and cast Time Walk (had an extra Blue) Sevinne + Flashback on multiple planeswalkers.
Omnath was also a Green card for Force of Vigor, which was pretty nice against the Golos Stax deck.
I favor this approach to a ramp deck that is all in playing Omnath, Locus Creation. While you might associate your build as a control deck, the presence of 28 or so lands really cuts down on your ability to interact with the stack and that will make things difficult in Vintage given the density of combo decks.
I also had experience playing Ugin in Vintage and this is not the metagame for it. The first times were during the Treasure Cruise era when I was trying to Drain a Cruise into an Ugin. The second time was a Rector Flash build with Narset unrestrict and a lot a Arcanist decks running around. Ugin does well against the URx decks that can't remove it while it just picks off all the creatures and walkers. Against Shops, Combo, and Dredge with Hollow One, Ichorids, Vengevines, it's not a good answer to the threats. Certainly not for the investment.
If someone argues that this isn't a true Omnath deck, I think a very reasonable approach is to try and explore Yorion next. But I did feel that Omnath gave a style of deck that had fallen out of favor a very real boost.
Adding Saheeli, Sublime Artificer to "The Riddler"
I believe "The Riddler" can benefit from adding one to two copies of Seheeli, Sublime Artificer for the following reasons:
After playing the deck, I found that the deck did not have a sufficient amount of creatures required to win the game in the vintage format when paired up with other agro decks. I looked at similar decks that do well with casting numerous spells in a turn and I found that Monastery Mentor worked well with Paradoxical Outcome decks as a win condition of overwhelming an opponent with Monk tokens. Though having Monastery Mentor in this deck would look good, I feel that the deck would not consistently produce enough white mana needed to cast Mentor even if additional white sources were added. Also splashing in white mana for just Mentor may not be a good idea. Adding Seheeli, Sublime Artificer to the deck would not take any alterations of mana sources. After testing this deck with Seheeli in it, I found that I had generated sufficient Servo tokens to win games against agro-heavy vintage decks.
Saheeli, Sublime Artificer's activated ability in combination with Time Vault is of great advantage. You can turn another artifact you control into an untapped copy of Time Vault and gain an extra turn with it. You can do this twice with a new Saheeli and you can even gain more than two additional turns by bouncing Saheeli with Paradoxical Outcome and replaying her. A couple of extra turns should be enough to win the game with this deck.
Saheeli, Sublime Artificer's activated and triggered ability in combination with Bolas's Citadel is an alternative win condition. Saheeli can generate a large amount of Servo tokens in this deck that can be sacrificed to Bolas's Citadel to make the opponent loose ten life. To make the opponent loose an additional ten life to win the game, use Saheeli's activated ability to copy Bolas's Citadel and use it's activated ability. There usually is enough leftover Servo tokens and other permanents available to sacrifice to the Citadel copy.
Saheeli, Sublime Artificer can be exiled to Force of will if you choose to do so.
Saheeli, Sublime Artificer can also be used to copy Hollow One on something like a Mox if needed or copy Blightsteel Colossus.
I believe that only one or two copies of Saheeli are needed since the deck has a massive amount of card draw.
Any feedback is welcome.