Two WebSocket clients (e.g. browsers) cannot talk directly to each other. So there will be an intermediary involved in any case.
WAMP is for real-time messaging. To be precise, WebSocket is for soft real-time. There are no hard real-time guarantees in any TCP based protocol running over networks.
Regarding publish & subscribe: a broker is always required, since it is exactly this part which decouples publishers and subscribers. If publisher would be directly connected to subscribers (not possible with 2 WebSocket client anyway, but ..), then you would introduce coupling. But decoupling a main point of doing PubSub anyway.
What exactly is your concern with having a broker (PubSub) or a dealer (RPC) being involved? Latency?