Sunburst, Purity, and Twobrid provide an opportunity to flexibly support many-color and monocolor in the same environment, with interesting deckbuilding decisions. I think they could work fantastically together.
Imagine first picking this card:
Twobrid Bolt {2/R}{2/R}
instant
~ deals 3 damage to target creature or player.
In the second pack, you have your choice between cards like the following:
Militant Mystic 2R
creature
haste
sunburst - ~ ETB with a +1/+1 counter for each color of mana spent to cast it.
0/0
Pure First Striker 2R
Creature
Purity - You may cast this anytime you could cast an instant if you pay only red mana to cast it.
First strike
2/1
Twobrid Catalog {2/U}{2/U}
Sorcery
Draw 3 cards, then discard two cards.
These three cards take you down different paths, but they don't lock you in, the way a card costing RWG or RRR or UU would. The Mystic encourages a base-red sunburst deck, in which you have enough mountains to cast the Twobrid Bolt for RR, but enough other sources for your sunburst cards.
The Purity card pushes you towards mono-red. If you stick to monored to maximize your purity spells, you can get enough playables if you pick up twobrid cards in other colors. But once you pick up those twobrid spells, you might want to splash to cast them cheaper, at which point sunburst cards become an option.
The blue twobrid card might increase the value of Sisay' Ring style artifacts that tap for 2 mana. Or you could switch to a base-blue deck, or even a red-blue sunburst deck.
This spectrum of deckbuilding choices I think would lead to an interesting, deep limited environment. However, that flexibility has the potential to lead to choice paralysis for new players. For that reason, I think we should stay away from twobrid cards with sunburst, or multicolor twobrid cards. Those make my mind melt just thinking about them. I also think we should include build-around uncommons that clearly belong in either monocolor or multicolor, but not both.