Co-op and multiplayer have become so prominent in gaming, and RE 5's co-op was exemplary from a mechanics standpoint but also just incredibly fun.
This is one of the core reasons why Resident Evil 5 deserves a remake, as there's not a game in the franchise more perfectly suited for today's era of gaming. It also introduced cooperative play, as players had to utilize their character's specific abilities to make it through levels and you had to be mindful of helping your partner to make sure you both survived. For those who don't remember, Resident Evil 5 carried over the overhauled game mechanics and slight shift to more horror action from Resident Evil 4, though it leaned even further into the action side of that equation.