What do I mean? A game that has no main story missions/quests and is progressed as the player does what
they want to do, everything unlockable such as abilities, weapons and customisation is locked behind an
experience system. Which is basically earned through doing the activities you want.
Quite innovative? ^^'
Minor troll apart, as the people said earlier, making a "huge" open 3d world tend to be impossible. It is not impossible, but it would require a equally huge amount of time, manpower and ressources.
If you really want to go that way, try to break down your idea on multiple mini-project or even prototype focused on one feature and see how long it takes, if your idea is really interesting, estimate the time to do it for real, etc...