Spencer is the character Resident Evil has been built on. Without him, everything would be different: the Mother Virus might have not been found, the mansion would not exist, Umbrella Inc. would have never been created... He's the most mysterious character, but we never saw him, though he's referred to many times in the story. .
I always wonder how is him, is he still alive, what are his intentions,... I think the creators don't want to introduce him in the games as a part of the action, but he'd be a great Boss, since he's probably under the effects of some virus or parasite, just like Wesker. There might even be a game in which his story is revealed: a part in the past, where you play as George Trevor trying to escape the mansion and one part in the present, in which you play as Wesker, who decides to confront his old boss :
It's just my opinion, what do you think?
(sorry for the bad English)
I always wonder how is him, is he still alive, what are his intentions,... I think the creators don't want to introduce him in the games as a part of the action, but he'd be a great Boss, since he's probably under the effects of some virus or parasite, just like Wesker. There might even be a game in which his story is revealed: a part in the past, where you play as George Trevor trying to escape the mansion and one part in the present, in which you play as Wesker, who decides to confront his old boss :

It's just my opinion, what do you think?
(sorry for the bad English)