(No joke) Future-proof defenders?

Registeel is the future! No offenders or defenders are safe...j/k, obviously;)

That might not be entirely possible. A change in the way base stats are converted from tye main games could weaken a lot of the current best defenders and strengthen a lot of other pokemon.

They could increase the weather boosted gym defender stats, so the buff is random.

for example during cloudy weather fairy types gets boosted up to 2+ times, this would make clefable one of the strongest defenders in the game, even counters would have difficult time.

in addition in an area on predominantly cloudy weather clefable should be a lot closser to blissey performance. Of course a weather boosted blissey will be stronger.

Milotic is probly up there I'd imagine.

I dont see other bulky waters beating it out any time soon and it's weak to Raikou/Zapdos/Grass. Two of those are legendaries that people arent guaranteed to have maxed out/invested in, good IVs, or even at all depending on the players, whereas your Blisseys you can get a Machamp from a raid easily. The only super easy counter will be Venusaur due to the event coming up so everyone will have one, but you'll likely see people if they even do max out work on their shiny which could have terrible IVs/levels.

Just thinking of ease of access to a counter, I think Milotic is probably up there.

Edit: Furthermore if you go with the Dragon Tail/Blizzard set, you can not only catch people off guard, but you can seriously dent counters if they dont dodge.

No other Pokemon comes close to Blissey's bulk

Sad fact of the matter is, that gym play is so (too) easy now. It doesn't really matter what is defending a gym - they all go down fast and hard. Only a difference of a few extra seconds and a few small potions...