Why would it be good?
We don't want the Half-Life 2 saga to end, which is a bit of a bugger given that the impending Episode 3 is going to be the last part. Half-Life 3 is years away, so we're going to need another way of extending our stay in the H-L universe while we wait.
The deeply textured backdrop of Half-Life 2, coupled with how much of a hoot H-L2: Deathmatch is, means that an MMOFPS could be a staggeringly exciting way of continuing.
How would it work?
We'd set it before Half-Life 2 - mainly because we don't know how Episode 3 ends yet - with the player taking the role of a Human or Combine foot soldier as the occupation of Earth continues. The game world would cover City 17, the outlying countryside and notable locations such as the coast and Ravenholme, but every area would be heavily fleshed out from the linear pathways available in the original H-L2.
Above: The finale of Episode 2 would have been devastatingly good in multiplayer. Go on, tell us we're wrong.
Both factions would have their own set of PvP and PvE missions, which would be written as side-stories and prequels to the main H-L2 campaign, filling it out with the details of the resistance which the player didn't get to see as Gordon. Valve could produce a truly epic account of what happened on Earth during Gordon's absence with regular updates to this game.
The Humans and Combine would have asymmetrical character classes and buffs reflecting whichever side they fight for. As a rule, the Combine would have better armour and tech, but the Humans would be quicker, and better at stealth and guerilla tactics. Both sides would naturally have their own safe areas of the map (Combine citadels or resistance hideouts) where missions could be picked up and parties formed.
We'd make a few changes to the traditional levelling system though. With real-time FPS combat forming the bones of the game, character progression would have to be based around skill as much as play-time, otherwise gameplay would become very frustrating indeed for new players. Thus, we would angle the EXP dispensing more towards things like shot accuracy, headshot percentage, damage taken and percentage of enemies killed in a mission, rather than simple grinding. Level-up perks would include things like equipment and attribute bonuses, as well as the number of team-mates recruitable for missions, which would in turn give access to the larger-scale, higher-level battles later in the game. Of course, gravity guns and HEV suits woudn't be unlocked until very late on...
Will it ever happen?
Steam has grown into a brilliantly connected community over the last few years, so Valve has clearly got a good basis to build upon. Though the MMOFPS is a largely untested genre so far, Sony's Planetside excepted, so we might have to see how games like Huxley and MAG fare before we see Valve take the plunge with it's biggest franchise.