AKA the general problems with Mega Pokemon, especially those of "weaker" or "niche" Pokemon who weren't given OP stat increase, Type change or new Ability. Not to mention all Mega Pokemon (except Rayquaza) need to give up their Item slot further holding back the lower tier Mega Pokemon.
Saying all this, I wonder if Mega Pokemon ever came back it should be redone. It was their first attempt at a Super Mechanic, I don't think anyone would blame them for a redo. Obviously don't toss out the designs, but I would take to heart what worked with Super Mechanics after Mega. Like one thing I would suggest without getting into hypotheticals: Allowing any Pokemon on your team capable of Mega Evolving to do so without giving up the Item slot. Give that flexibility which Dynamax and now Terastal have.
I think it's kind of unfair to look at Mega Evolution in the same vein as the super mechanics that came afterwards, because while it may technically be a super mechanic from a gameplay mechanics standpoint, the design philosophy behind it is completely different from that of future super mechanics, in that it's an extremely specific and personalized transformation power-up whereas future super mechanics tried to be more general, especially Terastal which is a completely general and universal power-up transformation with absolutely no specific/special version of it assigned to any Pokemon.
Mega Evolution is more in line with say, cross gen evolutions or regional forms in that the Mega Evolution itself is functionally an entirely new and different Pokemon in its own right. It's an individualized set of stat boosts to the Pokemon who gets it and oftentimes the Pokemon changes its ability or in some cases type combination, effectively creating a functionally different Pokemon from the base form. In some cases it may very well just be the mon but stronger, but oftentimes the Mega Evolved version of a Pokemon often functions completely differently, and different Megas often occupied different niches. You had some Megas which were offensively oriented and basically hard hitters like Charizard Y, Gardevoir, and Medicham, but on the other hand you had plenty of more defensively oriented Megas like Aggron, Sableye, and Audino who were more enhanced walls/annoyers that played more on defensive utility. And then you had different Megas serving different purposes across the tiers.
In that sense it's quite different from the later super mechanics and shouldn't really be looked at in the same thought process. Megas are basically evolved forms of the mon they derive from in their own right and like any Pokemon, some are bound to be less good than others: such is the nature of individual Pokemon in general.
The only "general" mechanic thus far that has really been particularly flexible in how it can be used is Terastal since Terastal despite being on the far opposite end of Megas in that it's completely general and universal, is functionally from a gameplay standpoint an in-battle transformation like how Mega Evolution is, except it takes being a general transformation and fully commits to it, in that unlike Megas which were individual stat boosts and ability changes, Terastal is oftentimes a simple type change and/or type boost that is activated by the transformation and the mon itself is functionally the same barring the boost to type and oftentimes the change in type. That said, that aspect of Terastal is very much a part of Terastal's individuality as a mechanic and what makes Terastal, well, Terastal.
Likewise, Mega Evolution stands out on its own because its unique trait is being a specific/personalized transformation that involves unique stat boosts to individual Pokemon who get it and oftentimes a fundamental change in the Pokemon. It fully committed to being a specified mechanic for certain Pokemon only and went in on making the most of it in that regard.
Z-Moves and Dynamax were functionally as battle mechanics effective nuke buttons. While Dynamax/Gigantamax "technically" was a transformation, it was one that lasted three turns, and the whole thing was effectively boosted nuke moves that had universal secondary effects (admittedly making for a rather boringly overpowered mechanic that I have openly expressed my dislike for), or in the case of G-Max a specialized secondary effect of a G-Max Move, and Z-Moves were effectively a one-use nuke button in a battle, not an actual change in the mon.
A bit of a rant but I feel like homogenizing super mechanics to make them similar to each other basically robs them all of what makes them distinct from each other, so I don't think "redoing" Megas would be a good idea. I do feel different people would feel differently about each super mechanic depending on what they want from a super mechanic, but each mechanic inherently had a different design philosophy from the get go and not all of them were inherently intended as "super nukes". They're all functionally different from each other.
Like Mega Evolution are basically designed as honorary new Pokemon and are an in-battle transformation, Z-Moves are basically a widespread one-use nuke button, Dynamax is basically "nuke buttons that cause different battle effects for three turns", and Terastal is basically a generalized in-battle transformation that involves type change/type boost.
Mega Evolution was very much effective at what it was aiming to do, which was to be a personalized buff for specific Pokemon, and fully went in on that, being new designs that were an evolution of the base mon's design (thus having new designs to stand out to fans), being stat boosts, ability changes, and in some cases a type combo change to effectively create a fundamentally new Pokemon. But like all Pokemon, there's always going to be some that are more effective than others: that's just the nature of Megas being effectively new Pokemon in their own right, and they're subject to that same power scaling that all ordinary Pokemon are subject to, but just about all of them were improvements to the original Pokemon.
So in that sense it really isn't comparable to future mechanics which tried to be more generalized. But those future mechanics shine best because they're more generic. Terastal is a completely general transformation mechanic that is designed to be a generic transformation that any Pokemon can use, and that's its individuality and defining trait as a super mechanic. Z-Moves are a one-use nuke button, and that's all they are. Dynamax in my opinion was fundamentally flawed from its conception but it took being a "nuke button move" to the extreme, basically its entire use was nuke moves with secondary effects, either homogenized universal ones or specific secondary effects for G-Max.
Really the main thing when we see a game with Megas again (ie Kalos or Hoenn revisit) wouldn't be to "redo" them but to simply expand on their use, perhaps by just having more mons that can Mega Evolve and getting more creative with the changes and stat boosts they do to future Megas whenever they revisit it. It's not meant to be a generic power-up and shouldn't try to be generic in any way, because the future mechanics work as general mechanics since they were designed as such from the onset but Megas aren't designed as such. If I wanted a generic power-up, I would just have Terastal or Z-Moves for that since that's their job as super mechanics.
Different super mechanics are different from each other, and that's how it should be if they want to keep making different ones.
