Mega Evolution – A game mechanic that perhaps sounded good on paper, but ultimately resulted a couple of issues, such as several Pokémon getting preferential treatment, several weak and barely used Pokémon getting Mega Evolutions such as Beedrill, Pidegot, and Medicham when several others were more deserving, some Mega Evolutions even making some Pokémon worse such as Mega Garchomp, and wasted item slots. Some Pokémon even became too broken as a result, such as Mega Rayquaza and Primal Groudon and Primal Kyogre. The best part of Mega Evolution was the in-game animations, the lore behind it, and the fact that some beloved favorites got their opportunity to shine and stand out, such as Lucario and Gardevoir. Its fatal flaw was actually more of a development issue, where too many useless Pokémon got the chance to do it rather than several Pokémon that deserved it. This resulted in many weak Pokémon such as Beedrill getting it despite still seeing no real use, and major legendaries like the Weather Trio getting it and unsurprisingly completely dominating the competitive scene in the most broken possible way.
Z-Moves – A game mechanic that was perhaps my second least favorite after Mega Evolution arguably because you basically waste a held-item slot for your Pokémon only to perform a single powerful move once per battle. In addition, some Pokémon received preferential treatment because of their status with signature Z-Moves. However, Z-Moves felt useful at times because they could turn the tide in battle. For example, if you needed to knock out a strong Pokémon with Focus Blast because you needed to hit a, for example, crucial Steel-type staller with a special attack but couldn't risk the low accuracy, you could use your Fighting-type Z-Crystal to turn your Focus Blast into the 100% accurate and stronger All-Out Pummeling for that one instance. Sometimes you might have also needed use your Z-Crystal to make a certain status move perform better, such as Z-Celebrate to boost all your stats before Baton Passing, for example. Its fatal flaw was a waste of an item slot just to use a strong move once.
Dynamax – A game mechanic that I know I mentioned before, but the main aspect that I didn't elaborate on was how strategic is it. Times exist when you must Dynamax to sponge attacks thanks to the doubling of your HP, or risk being knocked out by a strong attack. Times also exist where you must Dynamax and protect your Pokémon with a Max Guard at a crucial time if your Pokémon doesn't know a protection move, such as to stall Trick Room, or stall when your opponent would be knocked out by residual damage from Toxic. Times also exist where you need to Dynamax and easily set up an aspect of weather or terrain to make your Pokémon stronger and/or benefit the team as a whole, or even boost your stats and/or team's stats with certain moves such as Max Airstream, Max Ooze, and Max Knuckle. Dynamax allowed you to do so much in a battle that it was insane. However, its fatal flaw was, despite all the strategy it allowed, deep down, it was broken and needed a nerf. It allowed scrubs and noobs to autowin games, especially in 3v3 Singles and 4v4 Doubles strictly because of how strong it was, but the outcomes could potentially be vastly different if you battled the same person and disallowed Dynamaxing. Still, it was my favorite gimmick because when done right, you could make some awesome plays to decide the game.
Teralization – Another game mechanic that also sounded good on paper, but out of all the other battle mechanic gimmicks, it feels the most "optional" for lack of a better word. It's like, when battling with the other mechanics, instances existed where I felt that I needed to Dynamax, Mega Evolve, or use a Z-Move to win the game or knockout a problematic Pokémon that could destroy much of my team. However, with Teralization, I have won several battles without even having to touch the Teralization option. Don't get me wrong, Teralization definitely comes in handy when it does and is very useful in such instances, but many instances exist where your Pokémon can get the job done regardless without having to change their type. This especially holds true even beyond a competitive standpoint in a regular playthrough, where I noticed that I barely even used the feature at all. I had no trouble handling the NPCs when they all Teralized their Pokémon and the game created no situation where I ever felt that I needed to use this feature. Because of this, it kind of makes searching for 50 Tera Shards of each type to change your Pokémon's Tera Type feel like a useless grind in my opinion, and in a way, kind of makes those ridiculously difficult high-level Tera Raids feel a little pointless as well. Its fatal flaw is, at least what seems to be at the moment, a lack of necessity and seems more situational rather than staple.
To be honest, even though these game mechanics add some cool moments to the games, especially regarding animations and lore, I wouldn't be unhappy if future installations just gave us old fashioned Pokémon without the battle mechanic gimmicks.