Yeah, Deck Knight, you're completely right. I simply forgot Swampert's amazing list of resistances. So, Grass/Fire would work both ways: either dishing out damage, and surviving some attacks. Pretty much like... Gyarados? The difference is that the Grass/Fire would never get Dragon Dance, and the STAB moves are different and blahblahblah, but still.
It's a shame Grass won't win anymore, sadly.
Actually I forgot about a lot of Swampys resists:
Fire, Rock, Poison, Steel, Immune to Elec is the full list.
Granted that Poison is indeed lol to resist and Steel resistance basically means "I resist Metagross Meteor Mash," in a practical speaking manner.
And Grass types are underrated. They have several unique moves (or at least carbon copies of powerful moves) in Leech Seed, Aromatherapy, Leaf Storm, Leaf Blade, and various powders. Nature Power would be interesting in an enviroment where you can choose arena (Hydro Pump anyone?)
Some also have Growth, which I guess is bad compared to Nasty Plot and Tail Glow, but I wouldn't overlook a Growthed Overheat in terms of Threat level. There is also precedent for Grass types getting Explosion.
Really, Grass types do have some good unique options, and Fire mitigates a lot of grass weaknesses (only Steel does better, and that confers a 4x fire weakness) Its kind of like Water/Ground, where Water mitigates a lot of Ground's weaknesses and in return, Ground gives Electric immunity.