Natural Cure and
Regenerator allow you to switch into an attack or status and double back out if you need to -- this can be helpful if you're switching into a Keldeo Scald, for instance, but you predict the obvious Heatran switch.
Drought is your best option, allowing you to fire off monster Flare Blitzes for a brief window of time.
+1 252+ Atk Tough Claws Mega Charizard X Flare Blitz vs. 0 HP / 4 Def Keldeo in Sun: 342-402 (105.8 - 124.4%) -- guaranteed OHKO
Regenerator or
Gale Wings allow you to perform less-risky double switches or get priority Roost on your first turn. Not amazing, but still pretty solid.
Lightningrod is your best bet, allowing you to switch in on predicted Volt Switches and get a boost in damage. Otherwise,
Competitive is fun if people try to Defog on you or switch in an Intimidate user.
Beedrill really shouldn't be switching in, but if you have to,
Shield Dust,
Wonder Skin or even
Insomnia make you less vulnerable to certain types of moves. Still, Mega as soon as possible.
Intimidate is nice for some extra bulk, but the coolest option is definitely
Gale Wings. Due to how priority works, if you use Hurricane on the turn you Mega, it gains priority from Gale Wings and accuracy from No Guard, making you a godlike one-time revenge killer even before your boost in Speed!
Magic Bounce,
Levitate and
Insomnia are all solid choices depending on what you want to be able to switch in on, although your native
Magic Guard is still one of the better pickups. If you're feeling bold, run
Competitive and switch in on a predicted Defog for a free Nasty Plot boost!
Regenerator is still probably as good as it gets, but
Natural Cure is another option if you get hit with a Toxic before evolving.
Shield Dust could be useful, as could
Sturdy if you want to set up SD in regular form, but your native
Hyper Cutter is probably best to mitigate the no-doubt commonplace Intimidate.
Run
Water Veil to set up a DD in front a Wisp user like Mew, or
Water Absorb to safely switch into non HP-Electric Keldeo 100% of the time. Or just run
Intimidate, which you already have.
Defeatist is literally your only option outside of normal abilities. Strike fear in the hearts of your enemies by running this god-awful ability, or screw over the poor sap that tries to Trace you pre-Mega. Or just run
Pressure.
Intimidate raises your bulk even further,
Levitate gives you no weakness pre-Mega and
Volt Absorb,
Lightningrod and
Motor Drive all give you boosts for predicting an Electric attack. But wait, there's more!
Sap Sipper gives you an unexpected Grass immunity, and
Soundproof allows you to pivot into Boomburst and Hyper Voice users. Let's just say you have options.
While you're so slow this will hardly matter, you can actually run
Sand Rush to get boosted Speed on the first Mega turn, while also getting the damage buff from Sand Force! Otherwise...
Mold Breaker Stealth Rock/Toxic pre-Mega, I guess?
Download is for sure your best option, with the potential for a free +1 Attack before you even SD! Otherwise,
Sturdy and
Overcoat have their merits. Sturdy allows you to get off a BP even if you fuck up and switch into HP Fire, while Overcoat stops Spore shenanigans.
You're the only type of your kind -- better luck next time! Stick with
Guts.
Same issue. Keep
Sand Stream.
Storm Drain and
Water Absorb let you screw over Water types, while
Natural Cure,
Regenerator and
Sap Sipper also have use. However, the coolest strategy is
Contrary! Stay in regular form and fire off a few Leaf Storms, then Mega when you need to and keep all your boosts!
For rain teams,
Swift Swim allows you to get your Speed boost on the first Mega turn, which has always been a weakness for 'pert. Otherwise,
Water Absorb and even
Unaware have some niche uses as well.
Try
Filter -- your bulk isn't great, but you should be able to live some weaker Super Effective attacks and retaliate.
0 SpA Mega Venusaur Sludge Bomb vs. 4 HP / 0 SpD Filter Gardevoir: 153-181 (55 - 65.1%) -- guaranteed 2HKO
You...you want something better than
Prankster?
Magic Bounce -> Magic Bounce has its merits as well, but the first turn priority would be missed.
Someone mentioned
Magnet Pull which sounds hilarious if you can pull it off. Otherwise, opt for
Soundproof.
Justified allows you to take advantage of reduced Knock Off damage to get a boost, and no one wants to face +1 Mega Medicham.
All of Ampharos' options (see above) apply here, but
Intimidate -> Intimidate is definitely the silliest.
-2 252 Atk Landorus-T Earthquake vs. 0 HP / 4 Def Mega Manectric: 212-252 (75.4 - 89.6%) -- guaranteed 2HKO
Speed Boost is necessary, don't replace it.
Desolate Land makes a mockery of anyone that thinks you'll keel over to any Water attack. Shame it doesn't follow you post-Mega.
Intimidate,
Multiscale and
Delta Stream are all solid pickups for setting up a DD in the face of Pokemon that could usually destroy you.
-1 252+ Atk Life Orb Excadrill Rock Slide vs. 0 HP / 4 Def Multiscale Altaria: 88-104 (30.2 - 35.7%) -- 34.9% chance to 3HKO
252 SpA Mega Manectric Hidden Power Ice vs. 0 HP / 0 SpD Multiscale Altaria: 130-154 (44.6 - 52.9%) -- 23.4% chance to 2HKO
252 SpA Mega Manectric Hidden Power Ice vs. 0 HP / 0 SpD Altaria in Strong Winds: 130-154 (44.6 - 52.9%) -- 23.4% chance to 2HKO
If
Prankster is unbanned, Prankster -> Prankster solves Banette's biggest problem. Otherwise...
Levitate?
Mummy, maybe?
Use
Illusion to setup SD while appearing to be a totally different Pokemon! If you'd rather not play mindgames,
Intimidate and
Magic Bounce mitigate your bad bulk and allow you more setup opportunities.
Levitate and
Clear Body are the best you've got, though the latter definitely helps mitigate Intimidate spam.
Levitate and
Heatproof allow you to switch in to things that may have OHKOd or 2HKOd you.
Clear Body is decent to keep, as well.
Uhh...
Levitate.
Uhh...
Levitate (this was not a double post).
As a pure Normal, you have a TON of options.
Intimidate,
Sap Sipper,
Thick Fat,
Vital Spirit and even your native
Limber have varying degrees of defensive usefulness. Your best option, however, is
Download, which gives you a shot at a free Powerup Punch just by switching in.
Defiant is also useful for predicting Defog or Intimidate.
You'll probably want to stick with
Rough Skin, but
Levitate is a clutch option against non-Mold Breaker Ground types and also useful at preventing most hazard effects.
Snow Warning or
Soundproof are your options.
Pure Power um...
Justified is fine.
No ability will stop you from being the worst Mega.
Intimidate works best for CM sets, though.
Hell yeah,
Magic Bounce -> Magic Bounce! This allows you to do stuff like switch into Mega Sableye without fear of WoW or Tyranitar on the predicted Stealth Rock.