Hey C0RLoCK,
I agree with MustaKotka, leaning in to what each colour is good at will likely be the best idea for this kind of thing.
First, I'll point you towards edhrec.com in case you are not familiar. This excellent site will show you all of the available commanders, and you can split them by colour so you can take a look at each mono colours page and see if anything takes your fancy.
In terms of recommendations, I have a few ideas.
Black
====
Black looks to use anything it can to gain an advantage. It might use its own life as a resource, or sacrifice creatures for value etc. etc.
The best example of this is
K'rrik, Son of Yawgmoth. K'rrik is one of the most broken commanders WotC has ever printed, so might be too strong for the
balance you are looking to achieve.
Other good options are things like
Ayara, First of Locthwain;
Syr Konrad, the Grim or my personal favourite;
Yahenni, Undying Partisan. All good options for a generically good black deck.
Blue
====
Blue is most synonymous with control (counterspells etc.) but this tends to not translate well into multiplayer so probably not worth building around. Another aspect Blue works well with that does work well in multiplayer is Artifacts.
Emry, Lurker of the Loch or
Sai, Master Thopterist are both good options.
Urza, Lord High Artificer is another, but similar to K'rrik, is almost certainly too strong for Omnath.
White
====
Another thing White likes to do is "go wide" (create a lot more creatures than opponents and win through numbers).
God-Eternal Oketra or
Adeline, Resplendent Cathar are both good options for this strategy.
Red
====
DRAGONS!
OR GOBLINS!
Krenko, Mob Boss;
Magda, Brazen Outlaw;
Lathliss, Dragon Queen are all good options.
For slightly different strats,
Neheb, the Eternal and
Feldon of the Third Path both make for strong and interesting decks.
These are just a few ideas off the top of my head. Like I say, checking out edhrec.com is probably the best place to start. Once you've found a commander/commanders you are interested in, this site can also help you find specific cards that work well with the commander etc. Honestly, it's an incredible resource for building EDH decks.
One further idea that could be fun in terms of flavour but also help keep things balanced could be to build around a cycle of mono coloured legendary creatures. This is what I did when I built a similar thing. My choice was the Amonkhet mono coloured gods:
Bontu the GlorifiedKefnet the MindfulOketra the TrueHazoret the Fervent and
Rhonas the IndomitableThe resulting decks were all pretty well balanced and worked well against each other.
This obviously means putting Omnath in the 99, which might not be something you are interested in, which it totally fair enough.
If this idea is something you might
consider, let us know and we can help you find a cycle that you like the look of. There are a fair few to choose from.
Whatever you decide, I hope you have fun with it. Good luck!