You guys should know there is a thread for fan-made mega evolutions from which you can get some ideas.
That said, I probably should divide my answer into two branches: The fan-favorites likely to happen, and what I personally deem logical and would like to see.
Fan-Favorites:
Mega Flygon -- This one is pretty much a given. With so many people asking for it, I will be surprized if it doesn't get a mega (bet on Tinted Lens)
Mega Milotic -- Gyarados needs a counterpart on the special side. Naturally Milotic could always fit the part. The only issue is not to make it broken
Mega Gen2 starters -- Another thing certain. I expect Fire/Ground , Water/Dragon , Grass/Fairy.
Mega Froslass -- The only other branched evolution's opposite gender counterpart received theirs at the next game release (not shadow tag plz)
Mega Dragonite -- Other pseudo legends got their megas, and D-nite is much derpier and memorable than the other dragons.
My personal picks:
Mega Raticate -- I want it to be Normal/Dark and it's also quite possible since Pidgeot and Beedrill got one. It will also be at the top percentage.
Mega Kingler -- What about a Water/Steel king crab pokemon? No? Only me? Ok. I still want it tho...
Mega Toxicroak -- Its distant cousins received one, and it's such a cool design with good complementary movepool doomed to mediorcre .
Mega Gogoat -- Absolutely loved the goat's design, yet not impressed by the stats. Hidden potential, hidden power, work on it gamefreak.
Mega Krookodile -- Ground really needs a fast mega that can apply some offensive pressure without being too much of a gimmick. Cool too.