I was thinking about Keymaster Rogue vs Mystic of the Hidden Way, and it suddenly struck me that this could be an interesting keyword mechanic.

The creatures with Promotion themselves would in many cases not be very interesting. This is just an Angel of Mercy. But it hints at the way that there'd be two categories of commons here: those with promotion that are otherwise just undercosted french vanillas, and those with ETB triggers that you want to reuse. Just like Kor Skyfisher's ability was secretly an advantage in a landfall deck, Promotion could be a very powerful way to get extra use out of triggers like Mardu Hordechief, Blister Beetle, Nekrataal, Monk Realist, Sage Owl and so on.

"Price spike on Aluren, in aisle 3..."

It could say "another creature" if I wanted. That'd still let you chain 2 of them back and forth, mind. But, well, Aluren has plenty of other things that break it, and I don't think modern dev are going to be that bothered by it :P

Does the flavour of this work? The earlier creature, notionally a lower-ranked one like a Mardu Hordechief, will be being promoted to this higher rank -- which opens up a gap in the ranks for you to hire a new, surprisingly similar Hordechief?

Because I think the flavour is pretty good for Britannia. Promotion isn't a major focus of the series, but it is a theme that comes up from time to time throughout the 50 episodes. Suzaku in particular experiences a number of promotions, but so do Jeremiah and others. And I think it communicates something both of military hierarchy and shuffling people around almost bureaucratically.

Flavour works for me. (I did War Plan on the same theme; interesting we both chose ). Though there seem to me to be three problems:

First; this is flicker. Why white?
Second; for an effect that's weaker than Healing Salve is underwhelming. Yeah, it's repeatable; but it's not a shining example.
Third; you can promote a big thing into a small thing. Which is just a wording problem, really.

Did you just ask why Flicker is white? White is the primary colour of flicker effects, with blue secondary: see Momentary Blink, Turn to Mist, Cloudshift, Restoration Angel, Flickerform...

But anyway, this isn't actually flicker. It's self-bounce. Which is also white as well as blue: Emancipation Angel, Narrow Escape, Kor Skyfisher, and a few Planar Chaos examples.

What's more dubious is that if this were to be a Britannian mechanic as I proposed, that'd mean it'd be in black as well. Self-bounce or blink in black is very rare, though Theros did give us Rescue from the Underworld. It feels like a black version of Promotion would have you sacrificing the other creature, but that's a significantly greater cost (a card-disadvantage engine) and would need a lot of upsides to fix it. (Maybe "sacrifice a creature with CMC 2 or less, then return a creature with CMC 2 or less from your graveyard to the battlefield"??)

Green also does self-bounce (Roaring Primadox, Invasive Species), though in this case it's always as a drawback, whereas white has self-bounce both as a drawback but also at instant speed to protect its creatures.

But as far as I know self-bounce really isn't in black's portion of the pie.

I guess there was Skull Collector, as part of the Stampeding Serow cycle. One card is not very good precedent though.