All right, I have returned to cast forth my views.
First, MEGA MAGNEZONE! And now, duty finished, the next suggestion:
MEGA EXCADRILL
(No, I didn't draw Mega Excadrill yet, give me time~)
Type: Ground/Steel (Unchanged)
Ability: Arena Trap
HP: 110
Att: 165 (+30)
Def: 90 (+30)
SpAtk: 50
SpDef: 85 (+20)
Speed: 108 (+20)
OK, I'll be honest with you, this mega is designed for Ubers play, NOT REALLY FOR OU. Excadrill is already so awesome there that it needs nothing to buff it.
In Ubers, Excadrill is still a legit threat when paired with Tyranitar, as it outspeeds everything and has great power with a Life Orb. After an SD, even titans like Primal Groudon and sliiiightly weakened neutral Arceus will fall to it, and it has no problems cleaning up even +2 Xerneas (since those rarely run max speed). However, its main role of a Rapid Spinner suffers from one huge flaw: Zero Bulk.
Excadrill is one of those mons with high HP but paper-thin defenses. This Mega seeks to remedy that with a significant boost in both defenses. 110/90/85 bulk is not even bad by Ubers standards and excellent by OU standards (being much the same as Garchomp's really) as well as granting a big enough boost to its attack stat to be formidable, even when unboosted, in today's steroid-laced Ubers. This still does not match up to Life Orb in terms of raw damage output, but does come close without having to chip off 10% every turn. The speed boost is significant, as Excadrill loses out on Sand Rush on Mega Evolving. The given speed allows it to hit a nice speed tier that puts it above the myriad base 90s and base 100s in Ubers, but still below several Ubers staples like the Latis, Gengar, (M) Salamence, Mewtwo, etc.
The REAL change is in giving it one of the most dastardly of abilities: Arena Trap. Why Duggy the whack-a-mole gets it when a real mole doesn't is beyond me. This fixes that logical loophole while also actually helping Excadrill out very greatly: It basically debars things like Primal Groudon from switching out, thereby hopefully balancing out that thing's unholy level of influence in today's Ubers. In fact, it would finally be able to help the whale get back some relevance, since sniping P-Groudon can help P-Kyogre actually do something threatening. This also grants Excadrill excellent utility in Rapid Spinning or taking on a bulky Rocks role. Note that Excadrill can remain perfectly viable un-mega-ed as well, so a TTar + Drill can still work fine to remove fast things or boosted things before Excadrill mega evolves and traps the slower things.
And yet it is not game-breaking in Ubers. There are plenty of things that can outspeed it and Revenge Kill it (since almost everything in Ubers is on steroids and can end worlds) and its improved bulk is good but not fantastic, especially considering its lack of recovery of any form. It will also be limited by its moves to either Swords Dance or Rapid Spin; it can run both only at the cost of valuable coverage. Besides, there are many things in ubers that float, so Arena Trap is not as bad as, say, Mega Gengar's Shadow Tag.