Xenagos can easily become too powerful. We can mitigate this somewhat by giving the other colors answers and by having different decks fight for the key cards.
Xenagos’s Two-Colored Cards
[scryfall]Feral Animist[/scryfall] can be good with Xenagos, because you can pump the power, which works nicely with Xenagos’s ability. [scryfall]Relentless Hunter[/scryfall] is like a worse version of it in some ways, but its also better because it can get Trample.
[scryfall]Fires of Yavimaya[/scryfall] can be used to push that final few points of damage, but Haste is also a good ability.
[scryfall]Gruul Ragebeast[/scryfall] is just a creature I like. Its pretty expensive, which gives some time to the opponents of the Xenagos player, which might be needed.
[scryfall]Mina and Denn, Wildborn[/scryfall] are here because trample is good with Xenagos, but again, it also has another role. I want it in the set to encourage fighting for the Landfall cards that I’ll also be putting into the set.
[scryfall]Skarrg Guildmage[/scryfall] might be fun with Mina and Denn, because you can save your land, if needed. Also, Trample.
[scryfall]Zhur-Taa Druid[/scryfall] has potential in other decks as well, although it requires splashing. This is a big mana deck, so the Druid can help a lot.
[scryfall]Burning-Tree Shaman[/scryfall] has a special role in the set. Its an answer card to another deck, but its also fairly efficient creature.
This is going to be difficult. Red creatures generally have more strength than toughness and that’s a good thing with Xenagos, but again, I don’t want to make using Xenagos too easy, so I’m going to approach this a bit differently. I’ll make blocking harder.
I would love to put [scryfall]Malignus[/scryfall] here, but that would be too much. On the other hand, one of the things players love, is when they feel they can “cheat” the system, even if I have clearly built it in. I’ll use, but I’ll also probably take it out pretty quickly.
[scryfall]Archetype of Aggression[/scryfall] seems like the thing we want, although I’d prefer making getting Trample be a little more difficult.
[scryfall]Madcap Skills[/scryfall] goes well with the theme of making blocking harder.
[scryfall]Ire Shaman[/scryfall] is a card that failed to make a splash in standard (partly because of [scryfall]Abbot of Keral Keep[/scryfall]), but has Menace and gives a little card advantage. Also, I’m thinking of having a small Morph team in another color combination and this might be splashable.
[scryfall]Purphoros’s Emissary[/scryfall] gives Menace, but can work by itself as well. It does bring the general powerlevel down somewhat, but that’s fine.
[scryfall]Pyreheart Wolf[/scryfall] gives Menace to everyone. As a three mana 1/1, it seems pretty poor at first glance, but its very strong in the right situation.
I don’t want to push the Menace theme any more than that, so I have to figure something else out. Probably just something big, or since I really like the fatties from Theros, lets go with the red Monstrosities: [scryfall]Ember Swallower[/scryfall] and [scryfall]Stoneshock Giant[/scryfall].
I’m going in a different direction with the blocking here.
[scryfall]Tornado Elemental[/scryfall] has a nice way for getting damage through.
[scryfall]Outland Colossus[/scryfall] can’t be blocked by more than one creature and if you don’t, its going to be lethal pretty quickly. Of course, there’s the “cheating the system” aspect again, if you can give this guy Menace with one of the red cards.
Also, with so many big creatures, some ramp can’t hurt. Again, I /don’t want to go overboard, so maybe use some of the less splashy cards here, like [scryfall]Beastcaller Savant[/scryfall], [scryfall]Leaf Gilder[/scryfall], [scryfall]Nissa’s Pilgrimage[/scryfall], [scryfall]Golden Hind[/scryfall], [scryfall]Font of Fertility[/scryfall] and [scryfall]Peregrination[/scryfall]