I played Oblivion around 90 hours on Xbox 360. I remember not finding any new quests and everything just felt "finished". Unfortunately I wasn't able to complete couple of 'missions' (missing obligatory key, boss didn't appear etc.) because of the glitches. I even had to start a new game with a fresh character due to the missing dialogue option (Mages Guild quest line).

In any case I got all the achievements, you can check my profile https://live.xbox.com/fi-FI/Activity/Details?titleId=1112737745

Recently I've been thinking whether Oblivion still had a new content to be found. Your opinion?

Even though Elder Scrolls IV was probably my favorite game from that console generation I never bought the Shivering Isles expansion. It's also worth noting that I don't anymore own my copy and save files have been lost. I've made a jump to the PC.

I've played the game for thousands of hours. I still run into things I've never seen before.

There's no telling though whether you'd get anything more from the game. It's not really a matter of what's contained in the game, but of how you see it. I have an entirely different concept of it. I don't see it as something that can be "finished." My oldest characters still exist and still have lives in Cyrodiil, and I even visit them from time to time. I don't really see this as a game that I might "finish" - it's a world in which I create characters, who then go out and live lives, and I spend some time with them along the way. Any time I want to travel with any character, s/he's there, ready to go do whatever it is that s/he's going to go do, and I'll tag along. And maybe we'll even run across something I've never seen before, but that's not really the point. It's just spending time with those characters in that world.

If you can do that, the game is limitless. If not, then yeah - you're going to reach a point at which you've seen at least most of what's available (I really doubt you'll have seen it all - again, even after thousands of hours, I haven't seen it all), but you'll have seen enough that the new things will be relatively few and far between.

In either case though, you should try Shivering Isles. I'd rate it as easily the best expansion Beth has ever done, and it's arguably one of the best games they've ever done. It's good enough that it could well have stood as a full-fledged game of its own.

Don't fast travel, don't follow the roads and talk to everyone. That should set you up for a couple of years