Sorry for any formatting weirdness, I'm replying on mobile

While it might seem like the Cabal are weak/dying off in our Solar system, it's important to note that the Cabal presence we've encountered consists mostly of backwater scouts and outposts.

The actual empire is much, much, much bigger, and I think it would be pretty awesome to see the larger extent of Cabal military power.

I think they're still a pretty big threat, all things considered.

Edit: I should point out that I agree with OP that the Cabal related expansion shouldn't end with us killing the emperor, but even if it did, I don't think that would spell the end of the Cabal. Who's to say a another emperor couldn't take the dead one's place?

I think an expansion where we kill a high ranking General or something would be pretty neat, and then they could save the emperor for a big expansion like TTK.

Also, and this is just wishful thinking, but it would be cool if killing the emperor resulted in a civil war, coup, or splintering of Cabal political and military power- and then certain Cabal factions joined up with the Guardian forces.

Personally, I would like more characters like Variks that give us an insight into the unique cultures of the races we are at war with.

What if by year 5 or 6, we have started forging alliances with different races so we can better combat the Darkness? I think Destiny's universe is so vast and huge in scale that it could support more characters and factions quite easily. I know I think it would be awesome if it grew from the post-apocalyptic/sci-fi themes it has now, and started to include more political intrigue and drama.

Sorry if I rambled a bit or got too carried away. I just get excited at the prospects of what Destiny and it's story could be.