What do you think of giving Magcargo Desolate Land as a Hidden Ability? I KNOW it’s Primal Groudon’s but come on, that “18,000 degrees Fahrenheit” line should be implemented somehow! Plus, Drought was exclusively Groudon’s until it wasn’t, so I see no reason why Desolate Land should remain exclusive to Primal Groudon. It’d get rid of Magcargo’s double weakness to Water and jack up its Fire attacks. Add Trick Room support and we might make Magcargo usable then.
I don’t think I’m wild about it. For one, I would prefer to keep Desolate Land just for Groudon. It’s true that Drought and Drizzle were exclusive to Groudon and Kyogre until they weren’t, but I also think it matters that, even in generation III, Tyranitar had Sand Stream, so auto-weather as a concept was never exclusive to them. That, and I’m inclined to take the “18,000º F” line as the Pokédex just plain making stuff up. I’m also not sure it’s even enough – not alone, anyway. Magcargo’s still slow and has crap HP, as well as a double-weakness to Ground attacks. And unlike Drought, Desolate Land doesn’t leave a Sun effect in play if Magcargo switches out, so it’s actually less useful as a support ability, which as things stand is what Magcargo is best at (or… least bad at); his support movepool is surprisingly good, but he doesn’t really have enough damage diversity to go it alone since there are no good Rock-type special attacks. I suggested once that we give Magcargo an ability that absorbs Water attacks for a physical defence bonus (representing his lava body hardening into a rocky shell), and you could create an all-new ability for that or just make it a buff to Magma Armour, because immunity to freezing is not a worthwhile ability for anyone and the only other Pokémon with Magma Armour is Camerupt, who’s not in any danger of breaking anything (Mega Camerupt’s ability changes to Sheer Force). That definitely wouldn’t be enough on its own without increases to Magcargo’s HP and special defence (I should say further increases, because generation VII actually did buff his HP), but thematically I like it more.