This will never happen. The only reason Magnemite became Steel is because it wasn't a type change, but an addition. It was also very helpful to Magnemite since with Steel comes a lot of resistances.
Water/Dragon Gyarados would be very nearly broken IMO. It gains STAB Outrage and that would seriously hurt. Also its only weakness would be Dragon and a lot of stuff that threatened it would become useless against it. The only thing holding it back is Speed. People already banned Drizzle + Swift Swim because of Kingdra, which becomes unstoppable in the rain, and a Water/Dragon Gyarados would be pretty much a stronger, bulkier Kingdra. It wouldn't have the boost in Speed, but Gyarados has a better movepool and depends less on rain to sweep.
I have said it since the release of D/P and that is why Staraptor wasn't a Flying/Fighting type, It had no reason at all being a normal type! but in terms of any other pokemon I don't feel a type change would be in order, I know they did it with Rotom and depending on the Appliance it goes in depends it type which was alot better than how they had it before but I think if they change pokemon types now especially those from gen 1-3 would ruin the legacy alot of pokemon have already left.
The Magnemite incident people keep reffering to can't really be used as an example because we all steel was a recognised type/thing at the time gen 1 was released so that's why in gen 2 we saw a rise more defensive pokemon as well as the introduction of steel pokemon like Steelix and Scizor but I think Magnemite gaining a type wasn't something that was never going to happen it was something that was massively necessary at the time of the release of steel pokemon you have to consider that at the time steel pokemon were released they also introduced dark pokemon to the game.
For me what I fount strange was that Karen a gen 2 elite four member was supposed to be a specialist in dark pokemon but despite she uses two pokemon which are not dark pokemon and they're Vileplume and Gengar which made me think were both these pokemon originally supposed to be dark types? I would understand Vileplume becoming a Dark/Poison or Dark/Grass as the early artwork for Vileplume made it look dark and devious whereas with Gengar as it was in Gen 1 it made me think were they going to redo it's typing then decided at the last minute to keep it as a ghost? Who knows but if you look at the newer artwork for the Gengar line you will see that they don't look alot like ghost's they look quite solid and could be seen more as dark and devious rather than ghosts and scary.
Staraptor learns a single Fighting-type move. That's it. Meanwhile there are things like Pinsir which learn tons of Fighting moves (and since 1st gen even! Pinsir got Submission by level-up) and still aren't Fighting-type. It's the same as saying Fearow should become a Ground-type because of Drill Run. If Braviary, which is as the Pokédex states "the soldiers of the sky", didn't receive a Fighting-type, Staraptor has no reason to become Fighting-type at all. Might be what it needs to rise to OU, but instead of boosting Staraptor, GF should think about giving some tools to the other bird Pokémon like Dodrio, Pidgeot, Swellow and even Braviary, which nobody uses because they think it's "too slow for the metagame" but is a beast and should actually be the strongest of them all.
Karen got Vileplume and Gengar because they're nocturnal Pokémon and obviously to round up her team. Koga is the Poison trainer and has lots of Bug Pokémon, including Forretress which is Bug/Steel and is there in his team due to his ability to lay down Spikes.