That's fine. Got a democracy? You can have a veto.It wouldn't be a UN. Let's start a DN - democratic nations? But then the US would want veto power, along with several others I would guess.
We already mostly have the cooperation a DN would give us. Though isolationsts want us to weaken ties between nations.
The UN is, and should be, something different imo. And it is, though flawed, it does some good.