To begin, I feel as though this is what Mega Butterfree should look like. Credit for this image goes to Midnitez-REMIX on DeviantArt.
Long story short, its highest stat, Special Attack, is still low. It isn't bulky, it isn't particularly fast, and it can only really make use of its special attack by using Quiver Dance to boost it, assuming it takes a hit before getting to use it. However, the stat boosts from a mega evolution can bring this fan-favorite bug back up. For starters, we can try the simple approach to just adding 50 points to Special Attack and the other 50 to Speed, and we'd get a pretty powerful special sweeper with 140 in Special Attack and 120 in Speed. We could also take a page out of Mega Beedrill's book by taking 30 of the wasted 45 points in Butterfree's Attack stat and place 20 of them in its Defense stat and the other 10 in its Special Attack stat.This way, it could actually take a hit or two before going down, though a Defense stat of 70 and a Special Defense of 90 still aren't too impressive with an HP stat of just 60. Still, they work fairly well for a Pokémon designed to sweep on the special side.
Alternatively, I could imagine a bulky build, especially if they give Mega Butterfree a better typing. For this one, I'd take 30 from that Attack stat once again since, let's face it, few people would want to make use of its lacking physcial movepool. This time, however, I would add 55 points to the Defense stat and 35 points to the Special defense stat, thereby giving this thing a Defense stat of 105 and a Special Defense stat of 115. I would put the remaining 40 points in Special Attack and Speed such that this thing gets a Speed stat of 90, which is good, but still not considered fast, and a Special Attack stat of 110, so it can deal considerable damage while actually taking a few hits. While this would make it a better counterpart to the speedy physical sweeper Mega Beedrill, I honestly prefer the previous stat distribution since butterflies generally appear to have a light grace to them, and that goes very far against the bulk that this set has.
Overall, I would say that the basic ideas behind these stat distributions is what I'd imagine in a Mega Butterfree, and I like them both as they both make Butterfree relevant on the competitive side while not making it overpowered. Of course, that depends on the ability that this guy gets. Speaking of which...
Yeah, this is where Butterfree could see a huge lot of improvements. Looking at Butterfree's current stats as of the sixth generation... yeah...
My Ideas for Designing Mega Butterfree
I really like this design because there are obvious changes, such as in the colors and general body structure, though the changes are not too drastically off from the original design. Plus, this design seems to go well with so many different types and abilities.
While Butterfree's current typing of Bug and Flying fits, I feel as though this is a mediocre type competitively, and am tired of it because of how many Pokémon have this typing. Plus, it seems unnecessary. Sure, it looks like it flies, but so do so many non-Flying Bug types.
Many people seem to agree that Mega Butterfree should be a Bug and Psychic type, and I am one of these people. This typing mainly comes from the fact that Butterfree learns moves such as Confusion and Psychic.
In addition, I feel as though a Bug and Fairy typing would also be good on Mega Butterfree due to its design and the fact that many fairies are either represented by butterflies or bear great similarities to them. Competitively, however, the Fairy type wouldn't be too great, as it does not have many Fairy moves to use, and even with the Pixilate ability, it could only use a small handful of Normal moves. Still, I consider it an option.
When it comes to how a mega performs competitively, a good ability can make or break the deal. Looking at the abilities that Butterfree normally gets, it gets Compound Eyes normally and Tinted Lens as a hidden ability. In my opinion, these are both great abilities for Mega Butterfree for their own reasons, though I would imagine other abilities going great. The first such abilities would mainly play off of the majestic beauty often associated with butterflies, and make sense if it gains the Psychic or Fairy type. These abilities are Serene Grace, Magic Guard, and Magic Bounce, three of the best competitive abilities in my opinion. They would each be really fitting for this magical bug, and would help it be a good battler with its now good-but-not-great stats and occupied item slot. Also, if Mega Butterfree does get the Fairy type, it could possibly get the Pixilate ability as well, since it does not actually learn any Fairy-type attacks, even though Dazzling Gleam would really suit it. Finally, since many people find Butterfree to be really cute, I thought it could gain the Cute Charm ability if it doesn't gain any of the aforementioned abilities. While not as useful, this ability can still be situationally useful, though I would give it to the bulkier stat distributions, as Mega Butterfree needs to take a hit in order to use this ability.
And these are my thoughts. What do you guys want to do with the idea of Mega Butterfree? Please feel free to let me know what you think, and as always, keep being the awesome you that you are!