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It's funny because with Sap Sipper and the general lack of Grass types in high tiers it would be still garbage and irrilevant, not a secret it's still defeated by most of HO and stallish teams.
All it needs is a stats buff.
As for the Mega Ability that isn't a new one I'd go with Water Absorb or Dry Skin, Strom Drain is fine too.
Since rain is vintage no need of Swift Swim for it, to me Swampert always lookes like something that was ideal for the three water absorbing abilities. Especially seeing how hard walled it is by other bulky water, would fear only Grass Knot and random HP Grass with any of the mentioned abilities.
Other ideas I'd approve in this thread are: Regenerator (since salamender like), Sheer Force and Mold Breaker (usually a classic on big threating Pokèmon) and Adaptability, this is we're clearly aiming to make it extremely viable.
Personally, I'd like a slightly buffed in offense Swampert like +15 Atk +15 Sp.Atk but remaining also a bulky mon with a +35 or more in Def and Sp.Def taking the extra points from the Speed if needed. The key of this spread would be Recover as Egg Move or L.0 move through relearner.