Maybe I should weigh in at this point, now that the games have been released.
Mega evolutions can be treated as simple form changes. Stats, abilities and types are all things that can be changed between forms already. There's nothing about the Pokémon themselves that needs any serious work on in order to implement Mega forms.
The parts that would require some work are the technical aspects. These include a button which causes a Mega evolution (which only appears when it's possible to do so, i.e. holding the required item and haven't already performed a Mega evolution this battle), and making sure they revert back at the end of a battle (or does the Mega form only last while it's
out in battle, such that if it's switched out it'll return to normal even in the same battle?). Oh, and letting the AI perform Mega evolutions when appropriate. I don't anticipate these being hugely difficult to do, if you know what you're doing.
I don't know about you, but I couldn't possibly find a space in the current battle screen to squeeze in a new button. Another major issue is that there are no sprites of the Mega forms to rip from the games - I don't want to include fan-made ones.
Mega evolutions thus far only exist in one game (XY count as one game), and one instance does not a trend make. While I find it very unlikely that Game Freak would abandon the concept in future games (because when have they ever removed a Pokémon form?), by itself there's not enough to convince me to put any time into it. Plus I'm not that interested in the feature anyway.
And Shadow Pokémon are "Essentials"? Several minigames like the Unown ones are "Essentials"? If many people want (and isn't hard to implement) I didn't see problem at it.
This is possible to mega form be only in gen VI, but I doubt. And even if this happen, this should be more important that Shadow pokémon.
And I know that Shadow Pokémon are already at Essentials so there's no point of removing this feature.
You really can't use Shadow Pokémon as an excuse, because they were in Essentials before I took over. Obviously I'm not going to remove them, because why should I, but them being included in Essentials simply cannot be used as a measure of what kinds of features I would deem as "essential enough" to be added.
Not to mention that I don't solely add "essential" things anyway. I do what I want. In the case of the Unown (Tile) mini-game (and the Slot Machine and Mining mini-games which I also made), I made them because I wanted to make them, because I enjoyed them. I probably would have made them even if I wasn't the leader of Essentials; I just wouldn't have been able to add them directly to Essentials and would instead have posted the scripts as a resource.