You vs the guy she tells you not to worry about:
My favourite UR mini-theme at the moment is maximizing draw/discard triggers with Chasm Skulker, Drake Haven, TLG, and my pet card Alhammarret's Archive; I hadn't even considered Niv-Mizzet, and I probably shouldn't since I don't want two theme-specific UR 6-drops, but it's cool to think about. It's not hard to support as you want to run blue card draw and Looters anyway and the Tormenting Voice/Cathartic Reunion family of cards is popular around these parts, but the ceiling is very high if you want to go there (Attunement, Memory Jar etc).
A more demanding theme is library manipulation: the best Miracles are concentrated in UR(W), you can power up cards like Thought Scour and Predict, and effects like Brainstorm and Sensei's Divining Top become even better. This is usually found in RUG as green gives Courser of Kruphix, Oracle of Mul Daya, and Nissa, Steward of Elements. Riddle of Lightning is really fun if you can make it work.
Haven't people had success wit
Goblin Welder in URW? That's a deck I'm really curious about.
Welder specifically has been best in more aggressive decks. It's easy to slide it in early while applying pressure and it's never the most immediate threat so they have to deal with whatever's likely to beat them, and then you end up with an unkillable Smuggler's Copter or keep swapping out Clues or whatever for more threats. I thought it would be great in UR or BR decks as a reanimation piece, and it is, but it places structural constraints on your deck that make it weaker when you don't get the Welder nut draw. Welder itself is cheap but assembling Welder + disposable artifact + worthy Welder target + a way to bin that target takes time and cards. Most unpowered Cubes don't have the density of cheap artifacts needed to make it work so you need to have a comprehensive artifact theme. Starts like T1 Welder, T2 Cathartic Reunion, T3 Thopter Engineer + Weld make it all worth it though. More generally, the Welders are good in shells that have other uses for effects that fill your graveyard (either other reanimation like Feldon/Body Double/Mizzix's Mastery or flashback etc)
I've really liked URx artifacts so far; white has more artifact tie-ins, and WR artifacts is a deck itself, but black has more cards that draw me into the archetype: 1BR Daretti, Tezzeret AoB, Baleful Strix, Battle at the Bridge, Herald of Anguish. There are a ton of good token makers in UR, so my decks tend to rely on exploiting those (bounce/Blink, Tangle Wire/Smokestack, improvise, Emerge, Chief Engineer/Grand Architect).
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I tried to support the UR spells-based tempo deck for a long time and I hated it. In Constructed those decks rely on being able to run few threats as you want to run a lot of cantrips and filter spells that let you find those threats and then power them up if you have one. In Cube you don't have that kind of consistency so, if your first threat dies, you just flail around helplessly. When it does work, it can lend to intricate games but at the expense of any cute/interesting decks that typically get chewed up and spat out by aggro-control. The combo-control spells decks are my jam, though.