The coolest hacker-esque Pokémon ROM I've seen is InfiniteFusion [1]. Let's you combine Pokémon into original "fusions", in other words, there are over 200,000 Pokémon. Impressively well done and very fun with lots of user accessible settings like difficulty and degree of randomness. For example, this is the sprite you see in game if you have Blastizard aka Blastoise/Charizard fusion: https://www.fusiondex.org/blastoise+charizard/

[1] https://infinitefusion.fandom.com/wiki/Pok%C3%A9mon_Infinite...