Volbeat you're a Fiiiiiiirefly

 

 Some Pokemon have such a strong motif that it immediately becomes apparent what would have been different if they debuted in a later generation. With their design, movepool, and abilities, Volbeat and Illumise are just screaming to be Bug/Fairy types and it honestly shocks me that they did not get included in the Fairy reworks when that type was introduced. Especially shocking considering you could catch them in X and Y. Fireflies have such a strong link to fairies in the European tradition, we're going to let both of these mons be the fairies they always wanted to be.

Stats

Volbeat and Illumise have always been pretty lacking in the stats department, with only 400 for base stat totals when they debuted. A modest boost to 430 certainly didn't do much for them either. We'll be bumping them all the way up to 469 and shuffling their stat distributions around quite a bit so they can actually perform a bit in battle outside of Prankster shenanigans. 

Volbeat will be the offensively focused partner with a big boost to Sp. Attack and Speed. It will still want to boost with Tail Glow but now it will have an attacking stat that can actually utilize the boost. He'll also be getting 10 points to Defense to stay on the field a bit longer.

Illumise will be taking a slight hit to Speed to massively buff its HP.  Who needs Speed anyway when you have Prankster? She'll grab some small boosts to Sp. Attack and Defense too, but survivability will be key to her success as a support-oriented Pokemon.

Abilities

Prankster remains an incredible ability even after being nerfed against Dark types, and Volbeat and Illumise are the only Bug types that get access to it. The other abilities could use some work, of course, so you at least have an option to run something slightly different.

For Volbeat:

• Illuminate
• Tinted Lens
• Prankster

 It always struck me as odd that 1. Volbeat had such a gutter Sp. Attack stat when it always had Tail Glow in its arsenal and 2. that Illumise got Tinted Lens of all things when it is not set up to hit any kind of hard physically or specially. Easy fix here by moving Tinted Lens over to the newly offensive-focused Volbeat. 

I also can't in good faith take away Illuminate from a literal firefly, so we'll buff the ability instead. Scarlet and Violet meagerly tried to give it some kind of in-battle effect to match Keen Eye, but I think the nature of the ability should give it a stronger effect. Volbeat is lighting up the arena with its butt lamp, and everyone should be benefiting. Illuminate now gives a 20% accuracy boost to all Pokemon on the field, ally and enemy alike. Less extreme than No Guard but it affects every battler to set it apart. I like that this gives Volbeat something to offer on the team even if there is no chance for it to boost itself up with Tail Glow.

For Illumise:

• Supersweet Syrup
• Sweet Veil
• Prankster

Honestly, Prankster is going to be Illumise's preferred ability 90% of the time. As a slower, bulkier Pokemon with lots of useful status moves,why would you want anything else? Supersweet Syrup fits with the Pokedex flavor, at least, and can work well as an opening gambit to give a hard-hitting partner the ability to throw out powerful inaccurate moves. 

Sweet Veil rounds out Illumise's options by giving your team immunity to one of the most dangerous status conditions in the game. It might still be a little lackluster, though, so I'm toying with the idea of having it also wake up sleeping Pokemon when Illumise switches in. This would give it some synergy with Rest or with healing allies that got put to sleep when Illumise was off the field, but that might be a little too powerful. Not that Illumise really has the stats to make this a broken strategy. We'll see how testing goes.

Movepool

First off, easy change, Moonblast! They're fairies now, and they dance in the moonlight while also having Moonlight in their movepool already, let's make this happen. I'm also throwing in almost all the Fairy type status moves: Charm, Aromatic Mist (for Illumise only), Crafty Shield, Decorate, Fairy Lock, Floral Healing, Misty Terrain, and Sweet Kiss. 

The above is already a crazy buff, but I'm also adding Follow Me for both Pokemon because it fits so well with both design and their battle plan. You will find Bug types that are a lot mightier and tankier, but these guys are going to mess you up if you give them a chance with their Fairy tricks.

Their movepool is already rounded out with strong options such as Encore, Substitute, Trick, Tailwind, and Helping Hand. They are going to be a menace in the Double Battles format of Bugmerald if you let them get going. Only their mediocre bulk is holding them back.

Summary

I wanted to keep the identities of Volbeat and Illumise as Bug type pranksters that can fire off a multitude of priority status moves to disrupt the field and give opportunities to their allies. I think we've succeeded beautifully in that goal here, with a little extra bulk to help them survive more than a single turn in a fight and expanding their moves and abilities even further. While Fairy is one of the strongest types in the game, you'll be very limited in your Fairy type options in this game. All the more reason to consider giving these fireflies a chance to shine on your team.