Really? You really honestly, not in a cool hip cynicism to impress the kids who smoke way, think that for the first time ever they'll upgrade a post-Alara rare to mythic because they're too dumb to come up with two red mythics?
I'm wasn't 100% serious when I wrote it, but to be fair, they've done a really poor job with mono-red mythics so far in the Modern Masters line. I mean, if you break it down...
MM2013: Kiki Jiki (good), Ryusei (bad)
MM2015: Comet Storm (lol), Kiki Jiki (good)
So 2/4 mono-red mythics in the two Modern Masters sets are good and they're printings of the same card. Yes, I know Kiki isn't post-Alara as you said, but when I really look at Modern-legal mono red cards that could be mythic that could also use reprintings, I'm not coming up with much.
Here's a list of already-mythic cards I guess they might consider: Bonfire of the Damned, Past in Flames, Thundermaw Hellkite, Urabrask the Hidden, Koth of the Hammer, Inferno Titan, Warren Instigator, any number of Chandras, Tibalt (lol), the boring INN and DKA dragons, Stormbreath Dragon (if Theros is included, which I don't think it will be)
Here's a loose list of things I suppose could use an upgrade to a slot that aren't mythic: Goblin Guide, Through the Breach, Akroma Angel of Fury, Reiterate, Pyromancer Ascension
When I look at the list of mythics, it's just not a very exciting list. Some cards are more likely than others, obviously. If they do the praetor cycle then Urabrask is obviously a lock. Thundermaw Hellkite is probably your best bet for a creature, and people do love their dragons, so that might be an easier pick. Also some of those cards might be claimed by Eternal Masters, shrinking the pool even further. All things considered it's a pretty meh list. I'm not saying they're "too dumb" to pick something else, which I think is unfair of you to insinuate. And please tell me what I've left off that would be an awesome inclusion -- maybe I'm overlooking something. I'm just saying what's available just isn't super exciting and maybe bumping up Goblin Guide solves the "red mythics aren't exciting" problem these sets seem to have. That said, I'd love if they left it at rare.