Since the introduction of Mega Evolution we've seen a fairly wide array of Pokemon who can take on new forms, and in some cases ones which seemed unlikely to be given special attention after years of uselessness (like Beedrill and Mawile). For a while now we've been speculating on which Pokemon will Mega Evolve next, and some such as the pseudo-legendaries, starters and branch evolutions of Pokemon who already Mega Evolve seem like their eventual addition to the roster is inevitable.
However, not all Pokemon are guaranteed to eventually be given a Mega Evolution. Here is my list and the reasons why;
While I like Raichu, it seems unlikely that Game Freak would give a major improvement to Raichu when they've put so much work into giving Pikachu the major boosts. The Light Ball (and therefore Volt Tackle), the additional Cosplay moves, access to special moves such as Surf... these are all to make Pikachu viable while Raichu is a less marketable alternative. Apart from another anime episode to remind us that Pikachu wants to stay a Pikachu forever, I don't see what Mega Raichu would have to offer to a series where its unevolved form is the mascot.
To this day some fans just haven't got their heads around the intentional pointlessness of Farfetch'd. Farfetch'd is based on a Japanese saying about a bizarre and fortunate occurance, like a tasty duck which carries a leek for ideal soup. Farfetch'd used to be a one-off in the original games, a rare Pokemon you could trade a common Spearow for, but who lacked Spearow's potential to evolve into a strong Fearow. Even Farfetch'd's signature held item is just a plain old Stick. While there's the chance that Farfetch'd could have a Mega, it would need a highly beneficial ability to boost its stats beyond the 100 BST boost from Mega Evolving, because even that wouldn't make it very strong. And it's just not what Farfetch'd is about. Farfetch'd was created long before the competitive scene ever came to anything and simply isn't fit for that purpose. It's just a rare, peculiar little novelty Pokemon.
Marowak would certainly benefit from more attention, but a Mega Evolution would eliminate its Thick Club from potential use, much as the Mega Latis are weaker than their original forms holding the Soul Dew. If Marowak gains an ability like Huge Power which gives its attack a boost like the Thick Club could, then it may be possible.
This one's pretty simple: why Mega Evolve a Pokemon which transforms into another species before the first turn is up? One of Ditto's biggest drawbacks is the almost-certain reduction in HP compared to what it copies, and that's the one stat Mega Evolution can't boost. Plus Ditto relies on a held item to be of any real use, and a Mega Stone would take that away.
It would be downright bizarre if any of the Eeveelutions Mega Evolved but not all of them did. Their stats are designed to parallel and contrast each other and there are very likely more to come in future generations. They're never going to add a secondary type to the Eeveelutions either. They are among the most marketable Pokemon there are largely because of how similar and yet distinctive their designs are. Adding a Mega for every single would be unnecessary, especially when none of them have unique skills which no other Pokemon of the same type can have.
While I am a big fan of them and would like to see more expansion of the Porygon family (except in the anime of course- that's a long-lost cause) there are a couple of reasons why they're unlikely to gain a Mega. Firstly, they're man-made Pokemon and wouldn't fit within the mythos. Secondly, Porygon-Z is based on a corrupted file and therefore it doesn't make sense for its true, innate power to be released in the form of a "Mega" Porygon-Z... a more likely scenario is an alternative third form based on a vaccine, maybe with more defense power. But Mega Porygon-Z would be a major plot hole, not to mention how the anime would have to bury it.
Mewtwo was one of the first Pokemon to Mega Evolve, and one of only two so far to have two Megas. I don't see Mew being so lucky, again because of Mew's role in the mythos: it seems odd for a Pokemon from which all others descend to have a Mega form, especially when the new form wouldn't be able to have the across-the-board BST and would lose Mew's simple design. I think Mew is likely to stay as it is and let Mewtwo hog the Mega glory.
This is a lost cause. Firstly, why complicate a minimal design? Secondly, it will still have only a single attack. Thirdly, Unown are supposed to have a hive mind so an individual one Mega Evolving would make no sense... and oh yeah, IT WOULD STILL BE USELESS IN BATTLE. Unown are there for world-building, puzzles and tedious hunting. An extra hundred points won't help that, and nor should it.
Wobbuffet has nothing to gain from Mega Evolving. Its mammoth HP stat won't be boosted, it has no use for extra Attack, Special Attack or Speed. It has as good an ability as it could ask for in Shadow Tag. Higher defences would allow it to last longer, but also dish out less counter-damage. And it would lose its hold item. It ain't broke, so no need to fix it.
Plusle and Minun
These guys are a relic of a bygone age when double battles and abilities were new aspects of the game and they were a factor in teaching players how to use them, as their abilities were purpose-built for double battles. They're also meant to be cute, nearly identical and just not very threatening. There wouldn't be a point. You can only have one Mega on your team per battle, so having a Mega Plusle and a regular Minun together, or vice versa, would defeat the gimmick. Other Pikaclones like Pachirisu, Emolga and Dedenne also don't seem like top priority for Megas, but they're more likely to be put to use than the cheerleading twins.
Spinda demands enough work already without another variation and its millions of sub-variations. Spinda is like another Unown case; it's there for collection purposes and show off the more detailed, full-colour sprites the GBA had to offer back in the early 21st Century. I'll be very surprised if Game Freak ever decide to make Spinda even more complicated for the programmers.
Deoxys already has three alternate formes, and while a Mega would have a higher total and be stronger on the whole (not helping that minuscule HP though) I doubt they would do anything to make the Attack, Defence and Speed formes redundant, nor would a Mega Evolution be able to reflect all of Deoxys' facets. Deoxys was an early introduction to Formes, and is unlikely to jump on the latest bandwagon now. ORAS was the perfect time if they were ever going to introduce a Mega Deoxys, so I can safely assume it won't happen.
Rotom is another Pokemon who, like Deoxys, has enough Formes as it is. While it would be nice for the plain old Ghost/Electric Rotom to have stats comparable with its appliance formes, or even exceed them by Mega Evolving, Rotom already has enough variations going on as it is.
It's likely that Diamond and Pearl will be remade in a few years, but not Platinum. So while Dialga and Palkia will receive some sort of power-up, Giratina already has a forme change as a result of holding a specific item. I imagine that the Orbs will be retconned into evolutionary items like the Red and Blue Orbs in ORAS, but the Griseous Orb already transforms Giratina.
A few reasons for this: firstly, you can't boost all of its stats equally and that equilibrium is party of Arceus' design. Secondly, Arceus' look and capabilities depend on a held Plate, and a Mega Stone would eliminate the possibility of holding a Plate unless they pulled a Rayquaza and made Arceus evolve without a held item... which would be way too powerful. Arceus with a BST of 820, access to all the Plates and Judgment...? I don't think so. Thirdly, Arceus having an alternative forme just seems wrong to me, like with Mew. it just seems like Arceus shouldn't change that much, Plate-inspired colour schemes aside.
Tornadus, Thundurus and Landorus
Same as Giratina and Deoxys- they already have alternate formes. An improved BST would of course be a great thing, but I think these guys had their day in the spotlight and are unlikely to see another.
In a way, Black and White Kyurem can be seen as a sort of precursor to what would later be called Mega Evolution. There are of course differences, as rather than losing an item slot Kyurem absorbs a few extra offensive points from fusing with Zekrom and Reshiram. Like I said with Giratina, I imagine the possible eventual remake of BW will see a more powerful Zekrom and Reshiram forme for each mascot, but Kyurem won't change a jot.
Let me know any Pokemon you think have slim chance of a Mega in the pipeline.