Some people think they're dead, others think they became symbiotic with their suits and would die without them. The latter theory is supported by Prophet’s flashback meeting with Hargreave ("You turned them into dead men walking").
Assuming they are still alive, I see paths for two expansions. Both would be set during or slightly after Crysis 2. Psycho was recalled by the British government to combat the Ceph threat in the UK (there's a spear in London). Nomad would either be fighting the Ceph in Hawaii (another spear) or is deployed somewhere else in New York. An expansion could have both stories, one of them, or be two separate expansions to cover both.
Contribute any ideas you have for a plot or evidence that they are truly dead.
*The spear locations come from the game cut-scene Epilogue.