What are you people even doing
Like a third of existing Megas keep their abilities, and only 2 of 35 have a completely unique ability. So stop being ridiculous and giving every single thing a unique ability. Just try to make it viable.
My main issue with this is that most of the existing Megas were already powerful OU mons that would still be good with keeping their existing ability. Of the ones that weren't OU last gen, I see abilities like Huge Power, Parental Bond, Prankster, Mega Launcher (+ beneficial moves), Skill Link (+ beneficial moves), Drought, Magic Bounce, Mold Breaker, and -ate clones.
These are unquestionably top abilities, ones you often see in metagames when you can choose any ability for your Pokemon (until they get banned like Huge Power/Parental Bond or Wonder Guard lol). The only other top abilities that see play in metagames like Balanced Hackmons that aren't covered by existing Megas are Protean, Impostor (both seem to be signature abilities), No Guard + moves to abuse it, Magic Guard, and Poison Heal (both of which require status orbs for optimal use). If every bad Pokemon has to get one of those abilities, eventually there will be some Prankster/Regenerator/Huge Power Mega that's totally outclassed by all the similar options.
Even something like Scizor's rise from Gen II/III to Gen IV is the result of essentially the same process where someone at Game Freak said "Let's make this Pokemon OU-viable by giving it a new ability/new moves that are perfect for its current typing/base stat distribution," because otherwise it was just slow and not particularly hard-hitting. Similarly, think of Talonflame, which as a Fire/Flying ~500 BST Pokemon wouldn't be anywhere near OU if not for the exclusive, recently-invented ability it got. If there had been no such thing as Talonflame and someone proposed a Mega Fletchinder that got huge Attack/Speed boosts and the "new" Gale Wings ability, would it have been deemed ridiculous and been given something generic like Reckless instead?
Like I alluded to before, lots of these Pokemon are a far cry from being OU and need more than +100 BST and [insert top-10 preexisting ability] to get there.
No matter how you distribute the base stats, Mega Emolga is going to have essentially the same BST as base form Lucario (which isn't exactly tearing OU a new one as it is), and that's before factoring in typing, movepool, the inability to use Life Orb/Leftovers/etc, and the fact that it takes up your Mega slot. If it's limited to existing abilities, the best possible option for it from a competitive standpoint is something like Prankster, +50 to both defenses, and giving it Defog, and even that's of questionable viability.
Making some of these Mons viable is an uphill climb either way, and it should ultimately come down to flavor first and then the ease of which any new abilities can be implemented - I don't think one method is inherently superior to the other. It's just that abilities like Huge Power/Prankster/Fur Coat/Regenerator (extra Attack, Speed for support-oriented Pokemon, Defense, and HP, respectively) are essentially nothing more than additional buffs to the Pokemon's stats that don't count towards the +100 BST limit, so throwing them in without a really good reason just seems pretty meh.
