This would explain why we're still waiting for HL3.
Unfortunately, I think most folks would have been perfectly fine with HL3 being a continuation of HL2, myself included, so now we have to wait for Valves hubris to subside.
What could it look like instead? Anything! I think a Divinity-style RPG Half-Life would be heaps of fun. Or a hero-based RTS that still has Gordon Freeman centre stage. I think basically any genre that can tell a story would be fine.
Of course this guarantees disappointing a lot of people, but I feel there's a lot better chance of Valve producing something creatively worthwhile this way than trying to make a "vastly better Half-Life 2", which is what most people seem to be waiting for.