Could the Firebreathing mechanic (not the card) become a keyword?

Unlikely for numerous reasons (it’s activated, we don’t use it often, etc.)

Do one-off werewolves even need a mechanical focus? Most creature types don't, or lost their original focus over time. Elves don't just tap for mana anymore (or even tap much in general like they used to. Big sea creatures care less about islands than they used to. Dragons rarely have firebreathing. I can't think of many creature types that have a singular mechanical focus, now I come to think of it?

Imagine Slivers that didn’t grant abilities to other Slivers. (The two artifacts aren’t actual Slivers and thus the lack that ability.)

With R&D's Secret Lair, do Revised and earlier P/T modifiers (like Shivan Dragon's R: +1/+0) last forever? Or are the rules as they were incorporated into the card?

R&D Secret Lair makes cards play as originally designed, so firebreathing ends at the end of the turn as it did when Alpha came out.

Does "firebreathing but for toughness" exist and does it have a name?

White and green have done it, but the gameplay isn’t great. We never nicknamed it.

Relating to a Take Heart question from a while back: why does mentor target? It only seems to hurt potential interactions with cards like Take Heart or the firebreathing on Goblin Banneret, and confuse newer players, as it's not obvious at all why these interactions don't work.

Not targeting causes its own collection of confusing interactions.

Cost variations can be keyworded (see Scavenge and Cycling). You can have Firebreathing R, and Firebreathing 1R as long as you choose to format it that way.

No, we can’t. The result of cycling is always the same. Firebreathing is not. We often want to do higher mana cost for higher buff for mechanical reasons.

Is there any particular reason that Earth is decidedly NOT part of the multiverse? I always loved cards with real world flavor text (Firebreathing being the best), and the idea that Earth could be a part of the multiverse seems like a fun concept. Just not sure why there's a canon answer which limits the multiverse.

We don’t want a movie or TV show with people from Earth traveling to Magic’s worlds.

Could "Rootwalla Firebreathing" be done as red green hybrid?

I don’t know what that means, the two are different things. Rootwalla pumps power and toughness but only once per turn. Firebreathing pumps only power but can be activated multiple times.

In what Red is really good about other than direct damage and Haste creatures? It seems to me every other color does something Red does, but more efficiently.

Effects that Red is the top color at: (off the top of my head)- First strike (primary along with white)- Panic effects (“target creature can’t block”)- Temporarily stealing - Land destruction - Firebreathing- Rummaging (“Discard and draw”)- Impulsive draw - having additional combat phases- temporary mana production

Have you ever considered keywording Firebreathing ({R}: This creature gains +1/+0) or similar abilities?

No, there isn’t much value in it.