Brotherhood — I like this as an on-theme way to build WU aggro. White likes small creatures that can come out early, blue likes topdeck manipulation, they can get along well together with this mechanic.
Some enablers would be WU scrying, U surveil, and cards like
Not Forgotten or
Run Out of Town.
I will caution that since it's "shares a color", players will likely pick specifically W or specifically U for their core. When I played mono-blue devotion in Theros, I could play blue-white or blue-black cards too, but I wasn't going to play a nonblue card. You've similarly got a parasitic color mechanic that has its consistency maximized if I pick one core color.
Consider that if I'm playing brotherhood, I want to maximize my triggers. I will already only draw creatures at most 2/3 of the time, and I want to maximize the chance of all of those being useful. If I'm playing mainly blue, then playing only blue and white-blue creatures will do that. Any white creature I draw is a missed opportunity, so I just won't include any. If U is the stronger core, then mono-W brotherhood cards will just get ignored.
That seems a little undesirable, unless you want to encourage mono-color devotion building.
I suggest finding something to combo off that can exist in both colors. Tribe for example. Triggering off
all creatures, maybe. Or going in another direction, encourage players to build around creatures and enchantments and pick a term that encompasses that too.
Salvage — is really nice and removes some of the two-for-one aspect that comes from Auras, and it feeds off sacrifices. I like that!
If you make Brotherhood care about creatures and enchantments, then mono U starts being the color of auras in this set.
Burnout — works well with Salvage and creates a pretty clear theme in black. Arm your stuff, then cast spells, sacrifice it and burn it out and let something else salvage the tools you gave it.
I'd also be tempted to add Instants here to help people cognize their gameplay. The base set has a BR sorcery theme. This can interact with both instants and sorceries while still allowing the deck to focus on sorceries.
Given Salvage and Burnout sharing a color, I would unironically add an equipment that says
"At the beginning of your upkeep, equipped creature gains burnout and 'When this creature burns out, (something)'".
Incense — Flexible once-per-turn ability concept. It's nice. It requires several creature cards in your graveyard. I think it being in red-green means I'm going to be inclined to build red-green-black decks to dip into black's strong sacrifice outlet theme in this set. I recommend adding a black-green card that self-mills.
Pump up — Also a really nice effect. This creates a red-green-white theme of pumping up and lighting the censer, and using the casualties of battle to enable the next one. Make sure you add a card with a Censer ability that gives things +N/+N until end of turn.
Coming back to Brotherhood, I even more want to revise this mechanic. The parasitic nature of the color means it's a two-color theme that will make me build mainly mono-color decks. I mentioned Incense can build Jund, and Pump Up can build Naya, and likewise we can also see Grixis decks in here. But really, no three-color combo wants to use white or blue much in this set because white or blue will be busy being somewhat parasitic and exclusive to each other and you'll have a more limited card pool. Burnout decks get to benefit from
all of the cards in their neighbouring colors; the same can't be said for Pump Up or Salvage.
Build WU around creatures and enchantments instead, and you'll build a significant "aura matters" theme in both pump up + brotherhood, and in brotherhood + salvage.
