but I didn't really like the end as it was rather abrupt and left too many questions unanswered
That's because there will be a sequel

Rage2 was confirmed a while ago.
Most games are made with an end that could very well be the end of the story if the developers chose, but Rage was intentionally meant to make you feel like there must be more, so that you play Rage2 when it's out... which will be after Doom4 of course.
Anyway I'm lovin' the game, some little things I haven't liked, but overall it's been pretty awesome. I'm so glad, because Doom3 was very boring, and I just knew that Rage was going to be good, and I was right! Well, it's good for me at least,

I'll be on multiplayer as soon as I get a better connection though...