I feel like Crobat could use several more wings and some bad ass flames.
It could maybe, with the mega, do what it has always been trying to do but never quite got to do because other bulky status roosters did it better, thanks steel type and that other bat. 
Anyways, on to the suggestion.
Delta Crobat should be a poison/fire type bulky speed bat that specializes in dots and hots… The heal over time portion is going to be an optional one, giving this Pokemon the ability Poison Heal like they should have done when the ability came out. The dots is the primary focus though, toxic + will o wisp is going to be the foundational move set. There will be no debate of will-o vs toxic for this pokemon, it’s gonna be busted, tho trading ground immunity and all flying moves for a 4x ground weakness and not having it be a speedy sweeper is a good deficit for what I’m about to present~
Mega Delta Crobat
Type: Poison/Fire
We’re gonna make this thing die all over the place when an Earthquake hits. It’ll be alright though, if you out speed it maybe you’ll survive with… the…
Signature Mega Ability:
Bat Outta Hell
(or burning venom if it’s to be shared with non-bat exclusive pokes)
At it’s base, it’s also Poison Heal, we’re giving up Roost, Crobat needs this, but it also causes your poison to cripple the opponent’s Sp Atk by 50%, and makes the burns you inflict burn… badly, of which the same rules of being badly poisoned would apply, switching out or hazing negates the bad, but the special attack decrease will exist until Belled, clerics op.
It’s kind of an equalizer of the two damaging status ailments coming together and giving special attackers a piece of their mind while not giving up the ability to set-up against physical threats. On to the main course!
The Signature Move
Infernalize!!
A fire version of soak, dragonify, and medusa ray. I just think it would be good for Super Fang, Pokehunting, Crobat sets. Is it unncessary? Mostly. Is it satisfying to have alternative in world roles for your pokemon? Big time. Plus, I don’t think there really is a unique pokemon in this game with a unique specialization for the role of “Pokemon Catcher”. I’m pretty sure it’s still Breloom, Gallade, and Smeargle, so bonus incentive! But yeah, Everything else about this pokemon is already set up for tank busting and sweeper threatening. It doesn’t really need anything else.
oh, and as for the base stat Increases for the mega, i unno, prolly something like this?
HP 85 (unchanged)
Attack 80 (down from 90)
Defense 130 (up from 80)
Sp.Atk 80 (up from 70)
Sp. Def 130 (up from 80)
Speed 130 (unchanged)
Total 635 (up from 535)