Firefox crashed just as I was finishing this post! Sweet.
Well, here we go again...
If you refuse to kill Paarthurnax, how about random encounters where you could find him sitting about in the world, taking a rest. I'm sure even dragons need to rest sometimes, especially when they're trying to start a new movement within dragonkind. You could talk to him to change your shout buff, and/or have him offer to take you somewhere (eg, the Throat of the World, various places in the different holds away from people).
Adding this to the original post.
Another idea: after being inducted into the Circle, you'll occasionally run into skirmishes between one or two werewolves and Silver Hand members. While you still have lycanthropy, there'd be a chance for the werewolves to be passive to you. Maybe you could also sometimes run into Aela hunting in beast form.
Taking the first idea. Not 100% on the second--depends on how flexible Radiant Story is. It'd have to check a lot of things to make sure Aela should even be there. Besides, don't the Companions already go out on hunting expeditions? I think I've run into Varkas out in the wilderness a few times, and not always near Whiterun.
Also, personally, I don't think you should need to cure yourself to get requests to help cure random people. As long as you've already cured Farkas and Vilkas, I wouldn't see an issue with rumors spreading that you know of a cure. Though I think the cure should probably be changed, because running out to the middle of nowhere to get to Ysgramor's Tomb and fight a ghost wolf* will get old fast. This is also backed up in game, because using Ysgramor's Tomb was supposed to be a last-ditch effort in case Kodlak was dead before getting cured. For living people, the cure was supposed to be different.
Maybe after curing Farkas and Vilkas, you get an anonymous letter mentioning another way to cure lycanthropy, and after you check it out to see it's true (without having to cure yourself), then you start getting random requests for help.
* I also have to wonder why it's a ghost wolf you fight, and not a ghost werewolf. A werewolf is a daedra unto itself, which you take the form of during your transformation.
One of my goals for this mod is to keep the development side of things simple. To do what you're talking about, we're looking at basically adding new quests to the game; if someone else does this and is okay with it, though, I'd be happy to integrate it here.
I've already seen Legion/Stormcloak skirmishes.
I've also seen Thalmor/Stormcloak skirmishes.
Seeing as you already have a duel that requires a high skill in one-handed or two-handed, how about giving us mages some love and have one that happens when you have a high level of destruction or are the arch-mage?
@Lozzipop: I think that's already in the game. I've been challenged to mage duels on occasion in Whiterun, at least.
Clearly the UESP page is not comprehensive. It already lists mage duels, but it doesn't list (nor have I encountered) one-handed or two-handed duels. And while I've encountered plenty of Thalmor, Stormcloaks, and Imperials, I've never seen them fighting as part of a random event. Hopefully when the CK comes out, we can see what events already exist and what ones don't.
At OP: If you find the time to do this, drop me a PM if you need help with scripting and whatnot. If you don't have the time, I mighrt be tempted to do it myself.
I'll let you know.

Spoiler Or after you bring the 3 recruits to Delphine\or refuse to kill Paarth?
Maybe, but I'm not sure the effort of the extra scripting/testing quite justifies the payoff. (Not that this is a complicated way to do it; it's just a little more complicated than it needs to be, maybe?)