Hello!
I'm glad you're enjoying it! As you are in Wyndon, I really don't think you need to worry about your Grass coverage. Rather, I'd worry about expanding your Steel coverage, and having coverage for a Poison / Dragon type, and maybe a Fairy / Steel or a Fighting / Steel type. This all points to getting a great Ground type pokemon, rather than a Fire type. Ideally, you'd want one which is not also weak to Fighting type moves, so Ground / Rock and Ground / Steel are right out. After looking over the options, my top 3 for you would be as follows:
3. Flygon - Very cool Pokemon, has the advantages of being the fastest of the options. It hits hard, hits fast, has awesome coverage move options, and would likely fare the best for general usage. Its only weakness is that, being a Dragon type, it takes double damage from Dragon type moves, so if you wanted it to help make a certain battle against a certain Dragon easier, it would not be a viable option for that.
2. Mudsdale - It hits like a truck, gains defense when hit, and has enough bulk to take a few hits, which helps make up for its abysmal speed. Note that if you got one you'd specifically want one with Stamina, Own Tempo is comparatively sub par. It also learns Earthquake (the move you want these Pokemon using 99% of the time) the earliest of all the Pokemon. However it should be noted, it WILL go last, and thus it WILL get beat up on.
1. Golurk - It's still probably going last, but it has IMMUNITY to Fighting type moves, and takes 1/4 damage from Poison type moves, which are both Awesome attributes to hold for a Pokemon joining you at this stage of the game. Iron Fist helps with coverage moves by making them hit harder, and though it has the highest level to learn Earthquake, that can be worked around by getting it via TR. Plus let's face it - Golurk is cool. If I had your party and was at your stage of the game, it's definitely the Pokemon I would grab.
I hope that helps! Feel free to take it or leave it, Centiscorch is a fine option and if you aren't planning on using whatever Pokemon you grab vs the legendaries, it will help tremendously by expanding your Steel coverage through to the end. As a sidebar, I'm not sure why you're worried about Grass coverage when Falinks and Rapidash learn Megahorn, and Inteleon can (and should) learn Ice Beam and OHKO them with its superior speed.
