What happens after we defeated each boss of each plane/dimension? Where does trion go with stuff after that? Sorry not a lore person but kinda interested in it
Suddenly the Burning Legion invades Telara! Oh wait...
Well, jokes aside, its a newly established universe so everything goes, I'm sure it'll be very easy to fabricate new antagonists. After all, Rift's lore was never (imho) its strongest selling point.
A while back the devs talked about the possibility of us invading the elemental planes once we took down all the current big baddies. Last week I asked one of the GMs that routinely pops into world chat if that was still the intention and they said yes.
EDIT: And isn't it awesome that GMs just jump onto servers to ask everyone how they're doing, if they can do anything to help, and if anyone has a question they'd like an answer to? Yes, yes it is.
The Dragons are just returned to their respective planes of power (hehe) when defeated- they don't "die". Beside that there are a battery of villains associated with the cults on Telara and in/on the planes. Like there are bases of operation and so forth inside the planes- what do you think expert Rifts are? Lastly, time is 'fuzzy' in the planes.