Before anything, remind you all we have this thread: http://www.pokemonreborn.com/forum/index.php?showtopic=12041&page=1
In case you wanna read it and see if you like any of the ideas there. Regardless...
Hmmmm, if fake abilities can come into play, then I'll re-post mega shedinja (what? a man can dream...)
Mega Shedinja
Bug/Ghost
Heaven Guard
Hp: 1
Attack:130
Defense:45
Sp. Attack:30
Sp. Defense:20
Speed:110
Total: 336
Everything goes to physical attack and speed. Heaven guard works as a mix between magic guard and wonder guard: you can't burn, toxic or leech seed M-shedinja, neither can it die to the likes of stealth rock and spikes. However, it still takes one hit from a super effective move to take it out. With 130 attack and 110 speed, access to swords dance, hone claws and an actual utility for facade now, shedinja might actually pose a threat.
If fake abilities aren't an option, then....Mega Kecleon and Mega Dunsparce. Both have been deemed as garbage and abandoned throughout the years. So:
Mega Kecleon
Normal/Ghost
Protean
Hp: 60
Attack:110
Defense:70
Sp. Attack:80
Sp. Defense:120
Speed:100
Total: 540
Kecleon's leveling up movepool is full of normal and ghost type moves. With normal/ghost as base typing, Kecleon receives 3 key immunities, and a single weakness to dark (making things like knock off and pursuit a threat). Protean allows it to use its wide movepool to change to any of the types it can reach, while receiving STAB on anything it uses. While not as good as a protean user as greninja is, I think M-kecleon would be a nice touch for a pokemon that is so weak outside of the mystery dungeon series.
And...
Mega Dunsparce
Normal/Electric
Speed Boost
Hp: 100
Attack:120
Defense:70
Sp. Attack:115
Sp. Defense:65
Speed:45
Total: 515
I wanted a normal/dragon M-dunsparce, but the poor thing doesn't even get dragon moves. Instead, it has access to things like thunderbolt and return, both of which could receive good STAB from its typing. Speed boost could turn that awful 45 speed into something decent after a protect or two, and while it would certainly die to any earthquake/close combat, at least invested in HP it could survive minor hits, while hitting back with decent moves of its own.