Bandkrook Hi! I'm afraid that a very similar sub was proposed on the Discord
by War Incarnate, and we actually asked him to nerf it, which is the reason why his sub in its current state no longer lasts indefinitely. I'm afraid I'm going to have to ask the same of you - I don't think it's a good idea to give it an immediate, free, permanent Aurora Veil the first time it Mega Evolves and any time afterwards if it's on a hail team
(and since there are no Primal Kyogre, Primal Groudon or Mega Rayquaza in our mod, that still is permanent, haha), especially now that you've given it a better defensive type and given it even better offensive coverage than it already had.
Aurorus has worthwhile setup options and very good coverage, and I don't think it should need more than five turns of doubled defenses to prepare for a sweep, so the idea of letting it last
forever and letting it stall with Recover on top of that is definitely too much.
For reference, under Aurora Veil
(read: by default), this stat spread has better physical bulk than Zygarde-C and better special bulk than Blissey, which makes it the bulkiest Pokémon in existence on
both sides - that would be extreme even without Recover and setup access. I just think this Ability is definitely too strong to allow, personally. :'D
(If you do nerf it - whether you go the same route as War Incarnate in making it last five turns or not
it seems like he's okay with the similarity anyway because he liked your post - I'll still be happy to include it on the voting slate! I just thought I should let you know that it won't be allowed in its current state.)
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Mega Aurorus
New Ability: Refrigerate/Snow Warning ->
Diamond Dust
(During hail, Mega Aurorus is immune to Rock-type moves, including damage from Stealth Rock.)
Type: Electric/Ice
New Stats:
HP: 123
Attack: 77
Defense: 72 ->
102 (+30)
Special Attack: 99 ->
131 (+32)
Special Defense: 92 ->
122 (+30)
Speed: 58 ->
66 (+8)
(
621 BST)
New moves: Rapid Spin, Volt Switch
Description:
Aurorus has a lot of Electric flavor in canon already, owing to its aurora inspiration (auroras are based on disturbances in the magnetosphere, which is the part of space that's affected dominantly by Earth's magnetic pull), which also means having a ton of Electric-type and magnet-themed moves. With that in mind, I thought right away that it would be fitting to make it an Electric/Ice-type, and I settled on this as the most unique way to use that to its advantage!
Looking at its movepool and its existing stats, I decided pretty much immediately that I wanted it be a supportive tank, using its excellent dual STABs to free up its other two moveslots for support.
From there, I realized that it would suffer a
ton from its Stealth Rock weakness if I didn't do anything about it - my own favorite supportive tank is Araquanid, and it pretty much always runs Boots for the same reason. That meant I needed its Ability to be something that could patch that up... uh, without going full Magic Guard or Regenerator, that is.
I went with this idea in particular because I thought the Rock immunity would be useful to hail teams
(seriously, they need they need it so badly), and the hail requirement stays true to Aurorus's roots - but it also all but
requires the use of another hail setter if you want to make use of its Ability, because it's an Ability that helps the most in the long term rather than in the short burst its own Snow Warning will get you.
(I'm not really a fan of Abilities like Slush Rush on a Mega for a hail setter; I feel like those are generally either too strong without support for their first few turns in battle or too weak to be worth the team support they require after those turns are up.)
Once I suggested this Ability,
inkbug pointed out that the hazard immunity would make it an excellent remover and pivot for hail teams, which is how the two new move additions came up! With that in mind, Rapid Spin is probably what helps hail teams the most
(their only good spinner is Avalugg apparently... it's so sad), but other teams that run Aurorus will probably not be using it for hazard control anyway since it won't be protected by Diamond Dust, so it's worth bringing up that it also has
plenty of cool utility options in Thunder Wave, the ability to set Stealth Rocks of its own, Aurora Veil
(which it can use by itself at least once, even outside of hail teams) and Haze, plus Calm Mind if it just wants to be an incredibly bulky breaker.
With these many options in mind, I also tried to set up its stats to leave room for different options for EVs! You
could invest in bulk, and its defenses are definitely good enough to make that worthwhile; that said, its bulk is good enough that it can thrive as a spinner even without significant investment, leaving room to invest in Speed and potentially creep Pokémon as high as base 122 (!) after getting the +1 from Rapid Spin. This can potentially put it into a fantastic position to punish switch-ins, pivot out safely after clearing hazards, or even clean lategame with Calm Mind.
On the whole, I think the best part of this Mega Aurorus is its versatility - its wide array of support options, the freedom offered by its stats and the compression offered by its STABs makes it a unique choice with plenty to offer to any team, while its Ability makes it especially favor hail teams, allowing it to play an extremely valuable role for a team style that badly needs the help.
Mega Ariados, 
Mega Gourgeist and 
Mega Mimikyu will now be playable 
Mega Walrein
Mega Nidoking, 
Mega Tyrantrum and 
Mega Trevenant as counterparts to our existing Nidoqueen, Aurorus and Gourgeist!
-bitio, hematemasis, mf
-blue, me, hema
-pump, me, mf