Evolution is one of the key aspects of every mainline Pokémon video game, achieved after meeting certain criteria for each specific Pokémon. Mastering the evolution methods of the Pokémon franchise is required to complete the Pokédex and train stronger team members, and it's among the most exciting milestones for Trainers. Unfortunately, the video games limit this experience to occurring outside of battle, which puts them behind both the Pokémon Trading Card Game and the anime.
If players reach the level threshold for an evolution during battle, the Pokémon will not assume its next form until the battle has concluded. Since Pokémon receive experience points after defeating each opponent, it's common for them to level up before a battle ends - especially in longer battles against Gym Leaders or members of the Elite Four, but sometimes even against regular route Trainers. Whether the evolution would occur on Route 1 or at the Indigo Plateau, introducing mid-battle evolution could add a new level of excitement to Pokémon's somewhat formulaic battles.
Unlike the video games, the Pokémon TCG allows - and, in fact, heavily depends on - evolution while in combat with an opponent. While the TCG's rules are vert different from the games', this is one element The Pokémon Company could certainly adapt for the mainline series. Mechanics similar to mid-battle evolution have already been introduced in the form of Mega Evolution, Dynamaxing, and Gigantamaxing. Each of these has allowed for unique new strategies, and letting Pokémon evolve mid-battle could similarly invigorate the community's interest in creative combat tactics.
The Pokémon Anime Has Mid-Battle Evolutions All The Time
Like the TCG, many storylines in the Pokémon anime make evolution a key part of battle-centered episodes. Whether fighting a Gym Leader or thwarting Team Rocket's plans, the evolution of one of Ash's partners always turns the tide. The anime presents evolution as a big milestone in the relationship between a Trainer and their Pokémon, and allowing that milestone to be reached in the heat of battle only furthers its emotional effect. Being on the brink of defeat, only to have a Pokémon evolve at just the moment it needed, could translate some of the anime's drama into gameplay.
Aside from following the precedents set by the rest of the Pokémon franchise, letting players' Pokémon evolve mid-battle simply makes sense. If a battling Pokémon is in a high-stress situation and reaches the power threshold needed to give it an edge, why wouldn't it evolve right in that moment? Applying this same logic to Rivals and other Trainers could also introduce a fun new challenge, as the potential for opposing Pokémon to evolve during battle would increase the risks of allowing them to defeat any of the player's Pokémon. When Trainers in the wild or in gyms can adapt and learn, the challenge of Pokémon battles increases tenfold.
Nearly every part of the Pokémon franchise, including the new Pokémon Unite MOBA, allows Pokémon to evolve mid-battle, so the mainline games should allow it too. Game Freak has often introduced new features to the main game series, and it will likely continue doing so to maintain fans' interest. Mid-battle evolution could be just the change needed to spice up the next Pokémon games.