My B/R deck is currently looking like this:

4 Bomat Courier
1 Soul Scar Mage
3 Glint-Sleeve Siphoner
4 Scrapheap Scrounger
2 Lightning Strike
2 Abrade
1 Kari Zev
3 Heart of Kiran
2 Unlicensed Disintegration
2 Pia Nalaar
4 Goblin Chainwhirler
1 Aethersphere Harvester (tech vs. aggro decks)
2 Hazoret the Fervent
2 Rekindling Phoenix
2 Chandra, Torch of Defiance
2 Glorybringer

5 Swamp
10 Mountain
3 Canyon Slough
1 Cinder Barren
2 Dragonskull Summit
4 Aether Hub

Cards I still need:

- 1 Cut / Ribbons - kills various 4-toughness creatures (Steel Leaf Champion, Glorybringer, Nicol Bolas), turns into a mana sink / reach later. Those aren't that common so a single copy should suffice.
- 1 Kari Zev - high quality 2-drop. Doesn't crew Heart of Kiran but blocks Bomat Courier & Soul Scar Mage. Would run two copies.
- 1 Canyon Slough and 2 Dragonskull Summit - for obvious reasons. I'm actually not sure which one to craft first. It's important to curve out, but flooding is also a very real way to lose.
- 1-2 Soul Scar Mage - high quality 1-drop, especially in conjunction with Chainwhirler.

I have only one rare wildcard. Which card would improve my deck the most?

Smuggler's Copter

That is guaranteed to not help at all, since Copter is banned in Standard.

Think I'd go with the Mage.

I crafted mages until I had 3 of them.
My next target is Hazoret probably...

I love 2 Kari Zevs that card is great on the draw vs the mirror, most RB decks run just one Mage these days so maybe you should not put your Wild Cards there.
I do love running 3 Pheonix but maybe things have changed since I played it.
You should have one CUT//RIBBONS in your library after the AER AKH update... but I`m not sure. Remember its mana cost is at least 4 coz it adds both sides of the card.
Also, you don't run Ahn-Crop Crasher or Earthshaker Khenra those cards are amazing!

That Crasher just flat out wins games. I miss playing with him.

Problem with Phoenix is that it's competing with some other really good 4-drops: Hazoret & Chandra. One can only have so many 4-drops. I only have two each of the Hazoret and Chandra because they're legendary, leaving two spaces for Phoenix. As for Earthshaker Khenra and Ahn Crop Crasher, I'd say it's different shades of aggro. These cards polarize the deck towards aggression, and personally I don't like being so all-in. They're more mono-red cards than B/R cards imo. Also Earthshaker Khenra dies to Chainwhirler.

Copter is certainly not happening because ban (if it's unbanned it'd be insane, too).

Nobody thinks the lands are good cards to craft?

You can definitely go for a Dragonskull Summit if you want (I'd craft the third Summit over the fourth Slough simply because this gives you more options to open one or the other later).

I've seen both lists that run 1 Mage and lists that went up to 3; tend to still think 2 is the right number.

I played that deck quite a lot and 3 Phoenix was amazing precisely coz the Legendary clause, that also makes me wonder are you dropping your hand fast enough for Hazoret?

After Ramunap ruins were banned, this deck needed a way to use floating mana, that made room for a higher curve and cards like Glory Bringer, you run 25 lands and I think you are right on that number this deck is a beast when it curves out but without Ramunap ruins and Kenrha there is little to be done when you flood.

Sure Kenra dies to Whirler but a 4/4 haste creature that disables a blocker is a most welcomed sight at 6 mana.
The same thing with Crusher that guy is an integral part of the deck IMO, removing blockers lets you connect and if not at least it makes better trades.