Drifter
Red kind of has the space to do whatever it wants this set, since it has powerful burn spells and good common quality, meaning it’s well-prepared for lots of different games. I see beatdown and slower decks as both viable and potentially strong. Red’s burn spells are exceptional this set, because it has the tools to cover big and small creatures alike – there aren’t as many sweet spots for toughness against it for other colours to exploit. I suspect 5 is still a sweet spot, since only Squash hits that (and Squash will often cost 5 mana) so perhaps the 5 toughness creatures in other colours will be tough to handle (not that there are all that many).
I think slower Red decks will want to incorporate Boast, since it’s Red’s most powerful value tool. There’s kind of a weird dynamic with the slower Red decks, since Boast encourages them to have cheap and efficient tricks, but it doesn’t really have good ones, and tricks don’t generally go that well in slower decks. Still, removal will get the job done well enough, even if it’ll sometimes be too costly to remove something and then use a boast ability (though not with Frost Bite, and sometimes not with Demon Bolt!).
Boast doesn’t seem as good in beatdown decks, since it’s an inefficient form of value generation, but they’ll certainly still play the Boast cards with decent statlines. Aggressive Red this set looks it’ll be exploiting a bunch of common haste creatures to apply a lot of pressure very quickly, and then back it up with fliers and reach from other colours, or the uncommon burn sources it has access to. Boros seems like the natural home for aggression, benefitting from Red’s good cheap spells with second spell per turn payoffs, and some strong evasive Foretell creatures.
I fear one issue that Red has is that, though it has a Giants theme in theory, it doesn’t actually have very many and you’ll often have to venture into Izzet or incorporate Green’s changelings to have enough support. This wouldn’t normally be that big a problem, but a lot of its creatures this set are kind of understatted – most of them are 3/2s or 2/1s, or have abilities that hamper their blocks, so I think a big Red deck might want to get most of its creatures from colours with better blocking statlines on turns 3 and 4 (which notably Blue is good at this set).
Izzet seems like far and away the best colour combination for Red, since it has the most direct synergy in the form of the Giants deck and covers some of its weaknesses – such as that the slower Red decks don’t have all that much card advantage if they’re not getting Boast creatures through repeatedly (which will be pretty hard to do). Gruul also seems pretty strong, since Green will provide a much stronger creature core and better statlines, and has the potential to drive aggressive and slower Red alike.