Well my dream game would never be achievable any time soon because you would have to build like, 10 different engines and combine them for gameplay mechanics.
First of all, it needs an open world, huge sandbox, like, 3 times the size of San andreas per planet (or maybe twice the size of just cause if anyone played that game but me) 6-7 planets with completely different environments (one of which being earth) and multiplayer. At least 4 people, preferably up to 16. You're a very diversive customizable monkey (like chimpanzee size) with a jetpack, which you can utilize to go extremely fast as to travel around these sandboxes (as well as outer space) without too much drag. As far as weapons you have an arsenal of ninja gaiden/devil may cry style badass weapons that would never be practical in the real world, enough guns of different varieties to make gordan freedman hang himself, and superpowers, all interchangable and dual wieldable with different effects on each. There should be a level up system but not to the point where it gets grindy and boring. Movement with the jetpack should feel like a fun flight sim. Without it similar to prototype. As for combat, gun's only>First person shooter perspective, Powers only/ powers+guns>Third person shooter perspective. Single knifelike weapon+small gun or powers>stealth perspective. any other combo>Devil may cryesque action game perspective. Killable Destructable and eatable environments, buildings, Children, babies, monsters, Players, enemy monkey NPCs and civilians. Epic giant boss battles, Cool vehicles, awesome innovative weapons. Console controls/keyboard and mouse optional. If only it where possible.