the max speed is actually for other Gambits. Kingambit seems like it has a lot of room to grow but I'm skeptical that it will significantly reinvent itself like Dragonite, Ogerpon, and even Treads have.I think that as Kingambit is currently, it should be A+. However, it could definitely rise in the future as it does as some room for experimentation. For example, it actually has both thunder wave and taunt which could be very useful in theory. Also, for some reason it runs max speed (not including nature) and I have to ask why? What is it actually outrunning with max speed? Wouldn't it be better to run a bulky set with no speed investment so that you can live longer in the endgame? Either way though, Kingambit still has a bit of potential left.
While it does have interesting moves like Taunt, TWave, and even Stealth Rock, the issue is Gambit really needs STABs/SD/Sucker/Tera Blast to pose a significant threat. Kingambit is not an effective Taunt user when most of its counterplay is either offensive in nature (Great Tusk Headlong Rush) or outspeeds it (Moltres Wisp). The only relevant targets it could Taunt would be IDBP Corv (who can be beaten with Tera Ghost), Ting Lu to deny hazards (risky cuz of EQ), and Dozo (valid but Dozo is not even that common and Tera Dark Black Glasses has more use). More importantly, what move are you giving up? SD/Sucker are mandatory, so you're dropping either Kowtow or Iron Head, a difficult choice for not much value. Similarly, most hazard removers are actually great Gambit checks that would easily remove its Rocks. And Gambit is checked by most Ground types, so TWave would be difficult to spread when Lando/Tusk is always coming in.
As mentioned above, SD is too much to give up and that pigeonholes Kingambit into the bulky sweeper role without much room to diverge.
The truth imo is Kingambit has already seen an explosion of innovation and doesn't really have anywhere to go after this outside of new Tera choices (unlikely given how good its best ones are). While by no means is it bad or even predictable, it has too many checks for it answer with any one set. This leads to its signature "unfair" sweeps; it can never answer all of its checks with a single set but it can tech counterplay to a portion of its checks that let it run away with the game to the opponent's dismay. I don't think Kingambit will ever be anything less than great but aside from metagame shifts, its role and set is very much carved into stone (imo)