I think a metallic indoor training facility-type room would be pretty cool and different. Think X-Men's Danger Room. Baron and Dragon could be either holograms or robots (nice sparky and destructive particles, dented metal plates and pieces falling off as it gets weaker, little robot arms self-repairing it as it heals/resets, etc).
Perhaps it could be a joint Field of Justice created by the scientists and engineers of both Piltover and Zaun, with each side being themed after the other.
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Piltover's side could be cleaner, whiter, maybe more Apple-esque in design (think iBlitzcrank splash).
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Zaun's side could be dirtier, sharper-edged, not necessarily rundown but definitely not trying to hide the fact that this is a place to fight, kill, and be killed (perhaps there could be "stains" of the rusty brown/red type here and there on the walls and floor).
That goes for the shopkeepers as well, who could either be two different AI's or robots:
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Piltover's could be a light-hearted, positive, and maybe even semi-sadistic version of GLaDOS (as opposed to fully sadistic...think a younger, less corrupt GLaDOS)
"Oh, the Black Cleaver! Good choice, good choice! There aren't many better options to rip through your enemies and tear them apart limb from fragile limb, piece by delicate piece. Such brittle, frail lifeforms you all are... Well, good luck and have fun!"
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Zaun could have a more aggressive, sinister, maybe even masculine AI, maybe something along the lines of MCP from Tron, or SHODAN (not exactly masculine but she is sinister).
"Mm... Deathfire Grasp, I see, yes, a wonderfully efficient and compelling item, deadly, with a hint of surprise... Your enemies will never realize what slaughtered the lot of them. Very efficient, very compelling..."
Or two different scientists behind a blast window with all of the gear and shop items in storage boxes or whatever behind them, one from Piltover, the other from Zaun:
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Piltover's scientist could be a colleague of Heimerdinger's, similar in extravagance and character. Either that, or some lady dressed professionally and with a pretty face assigned by Piltover to assist Champions with item selection.
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Zaun's scientist could be an old, grumpy man (I picture David Bradley as Argus Filch from Harry Potter) who just wants the shopping Champion to hurry up and choose something and then get lost (if they were an enemy of Zaun or unrelated), or maybe they have a deep, repressed love for fine technological work, but they're a retired (brilliant) engineer which explains why he's there in the first place, and when someone comes to his shop that is a technological marvel or has something of the sort (Viktor, Heimerdinger, maybe he can comment on how brutish, barbaric, and "backwards" Freljord champions are with their big swords and bows and other old technologies) he still wants them gone, but he doesn't mind them so much if they took their time (just so he can look at the work done on their gear and such and admire it). Either that, or maybe some punkish and/or crass young biochemist lady with a sharp tongue. Think Zaunite Vi, maybe more rude.
Or It could change one match from an AI/robot to a living faction representative.
As for what the brush would look like in this "Danger Room"-esque SR...perhaps that could be a special hologram? I would suggest probably only on the Piltover side, but I understand there would be some consistency and readability problems there, so perhaps holographic brush all around (maybe a deep, bright red to stand out? Or a vibrant but dark green?).
I dont know, I think it'd be pretty neat, and a good way to get more lore out there for the players to soak up. :P