Super Smash Bros. Ultimate DLC: 7 Pokémon We Need To See
2. Plusle And Minun
At one point, it seemed Plusle and Minun were considered as playable characters in Super Smash Bros. Brawl, as data filings for 'Pra_Mai' (likely short for the Japanese names of Plusle and Minun, Prasle and Mainan) was found in the game's code.
Maybe for Ultimate - its namesake alone comprising as many fighters as possible - we could get the duo playing similar to Pikachu, but with the Ice Climbers' team-up mechanic.
Whilst they aren't as popular now as they were back when they were likely considered for Brawl, Minun and Plusle are still the mascots of the third generation of Pokémon games - a gen that needs WAY more representation overall.
It isn't too far-fetched to think that Sakurai would want another fighter to function similar to the Ice Climbers in Smash Bros, and it seems that he has had his eyes on Plusle and Minun since Brawl. We can only hope.