I think they could very well bring Venom back without too much difficulty b/c of how they killed him off. When the explosion occurs, Eddie is vaporized, but Peter could not have seen that b/c the blast forces him to look away. So, Peter assumes Venom's dead and most people would. It's a bit of a stretch, but conceivably the symbiote could've pulled away with Eddie just before the explosion. They still would've been hurt, but would've survived. As for how they could explain away Venom's death, it would be a simple retcon of what was shown on screen was only there for emphasis for the viewer and not a part of continuity as Peter didn't technically see Venom die.
A Venom spin-off could be very cool if it was handled correctly. Much like how Venom's personality changed a little from being completely psychotic and only looking for revenge against Peter to being a type of "protector of the innocent" and occasional ally of Spider-Man. If the storyline focused on the duality and inner conflict of Venom/Eddie between vengeance for a perceived wrong against him and his twisted view of righteous justice as well as had an interesting villain (like Carnage or a reinvented Toxin), I think a spin-off flick could be quite interesting. And if all the right pieces are put in place, I think Topher should definitely consider reprising his role as he may never have a bigger role than Venom.