Carbon Black/Rose

Carbon is the son of Ruby Rose and Mercury Black, but his last name changes depending on the AU, so there's the Black AU and the Rose AU 

Black means Merc stayed with Ruby and helped raise Carbon, being a better father than his own was.  Carbon grows up with all the love and support he'd ever need, able to do anything he sets his mind to.
Rose means Merc's relationship with Ruby was a bad one.  Carbon was born into a home of fear and hatred that boiled over when he was just becoming old enough to think for himself.  Merc left Ruby a poor emotional wreck, leaving Carbon to be her support, which is a lot for a kid.  Instead of making his own symbol, he adopted Ruby's when he changed his last name from Black to Rose to erase any trace of Merc from their lives, which is kind of a heavy burden for a kid to uphold.

I'd like to think the Rose AU ends Mariner's Revenge style with him and his dad locked in a room and only one coming out alive.  Merc gets his ass beat while Linkin Park's "Breaking the Habit" plays in the background

I don't have a whole lot of personal info about him yet aside from his weapon being shoot boots with blades on them in the Black AU and Ruby's scythe in the Rose AU.  His semblance is perfect/enhanced vision (like X-Ray and being able to see far away n such)

Also in this AU, Grimm are still there, but no one in the main story has died, meaning Penny, Pyrrha, Torchwick. and Adam are all safe

Relationships in the AU include:
Ruby Rose/Mercury Black
Yang Xiao Long/Jaune Arc
Blake Belladonna/Sun Wukong
Coco Adele/Scarlet David
Cinder Fall/Sage Ayana
Weiss Schnee/James Ironwood
Nora Valkyrie/Lie Ren
Emerald Sustrai/Fox Alistar
Velvet Scarlatina/Roman Torchwick
Penny Polendina/Neptune Vasilias
Pyrrha Nikos/Adam Taurus
Neo Politan/Yatsuhashi Daichi

(And no worries, everyone is at least in their 20's before they start getting together