So GF has given us Mega Glalie and Mega Steelix. This is very interesting.
Glalie was never a very good Pokemon. Not only did its base stats suck, it was a huge pain to get. You had to go into Shoal Cave at the right time of day and get through the maze of caverns (while being assaulted by Golbats!) to reach the lowest part, where Snorunt would appear. But for a Pokemon that you had to go so far out of your way to catch, it frankly sucked. Not only did it take forever to evolve at level 42, but after evolution, its base stats, at 80/80/80/80/80/80, were terrible. Although it gets Crunch and Ice Beam, Glalie was a perfect example of a Pokemon that was doomed to be nothing more than dex fodder. Then generation IV introduced Froslass, an alternate evolution of Snorunt, which was the final nail in the coffin for Glalie as she does everything Glalie can do better. If I have to say something nice about Gen III Glalie, I can say that Glacia had two of the mutant... ice... face... things, which were sort of jokes.
Now we have Mega Glalie. If Charizard, Mawile, Kangaskhan, Pinsir, etc. are any indicators, a Mega could save Glalie from obscurity. But we know nothing about it other than its design, so... speculation time! Judging by Glalie's appearance, it looks like it might be Ice/Rock or Ice/Dark with Strong Jaw. This would suggest an increase in Attack, and Glalie has Ice Fang and Crunch to use with Strong Jaw. Ice/Dark seems more likely, since it doesn't have any Rock moves to use. Mega Glalie is pure Ice and has Refrigerate. That... makes absolutely no sense concerning its design, but whatever. I don't see Refrigerate being too useful, since Glalie's Normal-type movepool consists of... Return, Hyper Beam (lol), Giga Impact (lol), Round, and Explosion. I guess STAB Refrigerate-boosted Ice-type Explosion is pretty powerful, but still, it would take a fairly large Attack boost for it to matter.
*inb4 TrollFreak gives it no Attack and a ton of SpA*
As for Steelix, it has always been a bit of a fan favourite. Back in Gen II, it was a defensive titan, boasting a staggering 200 base Defense, not to mention its rocking (see what I did there?) design. But Steelix fell victim to the power creep, and its low HP and SDef made it unfavoured. It also had to compete with Skarmory, who had immunity to the omnipresent Earthquake, no weakness to Fighting, and reliable recovery in the form of Roost. Skarmory was used over Steelix as a defensive Steel-type, while Steelix fell into the lower tiers. In-game, until DPP, Skarmory could be used right off the bat when you caught it, while to get Steelix, you had to find a Metal Coat and someone to trade with.
And now we have Mega Steelix. First of all, its design is top-notch. Many of its Dex entries state how it is as hard as diamonds, and it looks like the spikes protruding from its body have become actual diamonds. Since its ability is Sand Force, it looks like Mega Steelix will become a bulky attacker instead of a physical wall. This is the kind of Mega I prefer, where the Pokemon's role in battle completely changes. As for the stats...
A Level 40 Steelix with 0 IVs, 0 EVs, and a neutral nature has stats of 110/73/165/49/57/29. The first Mega has stats of 114/107/170/52/82/31. Since the HP is different, we know that this Steelix has some IVs. The increases in Attack and SDef are big enough to suggest that Mega Steelix will get boosts to those stats. In the second picture, the stats are 110/111/197/51/81/39. We see the same boosts in Attack and SDef, but there is also a boost in Defense. Hmm...
EDIT: According to the pics posted above, Steelix does indeed get increases in Attack, Defense, and SDef.