Yeah, the closest thing we got is stuff like Radical Red with a portable PC, enabling you to switch your team up anytime. There's also the PartyPlus plugin for Essentials v19 but i havent seen any fangame use that plugin, which is a shame
As the title suggests, are there any fangames or romhacks that use an increased party size? The only one i can find is Emerald Eight which does exactly that, increases the party size to eight. Honestly due to there being thousands of Pokémon now, a larger party size would work for more varied...