Had several variations on this in the mountains on the French side (stews of various meats) of Basque country, about this time of year in fact. I can tell you what the restaurant, at which we ate three times -- it was our hotel -- did, which I assume is typical, but I don't really know. First, there was always a root vegetable involved...one night in the stew (it wasn't potato, but a word neither Theresa nor I knew; turnip is what it tasted like, but it could have been something else), the other nights either on the side (classic roasted/semi-smashed/fried-to-finish potatoes dusted with piment d'Espelette) or underneath (straw potato cake, still glistening with the local butter in which it was fried). Otherwise, the stew was always very simple but rich in flavor, as of course any traditional mountain dish would be. Not too "interesting," as such.
If you want to stay Basque, apart from peppers the other major regional signature product (on the French side) is cherries, and so how about some dried cherries in the stew? I think they'd go great with lamb. In fact, the lamb-cherry-piment d'Espelette combo is making me hungry just thinking about it.
If you cross the border, you can start to introduce some variations on chorizo, but now you're taking the stew in a different direction. I'll bet Victor has some thoughts on how the Spanish side would do this, though I can say that the two times we had a plain stew (mostly, we ate pintxos or at restaurants that weren't going to serve anything so prosaic), once in Haro (OK, not Basque) and once just above Bilbao, they were also very, very simple: meat and liquid. The one in Haro was just reduced animal liquids, not really thickened at all, and reasonably heavy on the black pepper. (Victor, does that sound like the baby lamb dish at Beethoven, or am I misremembering, as I'm not looking at my notes at the moment?) The other was an extremely thickened, hearty stew that any traditional French cook would recognize, though this was veal and not lamb. Again, almost nothing but meat and the thickening agent.
If you want to go off in an entirely different direction -- far from Basque -- I did lamb the other night this way: sauce of roasted and rehydrated peppers of various types (I used fresh anchos and dried guajillos, and fair amount of both), a little red wine vinegar, toasted sesame seeds (a lot), roasted garlic, a little thyme, and a little Mexican oregano. Remove the pepper seeds, pure everything with the rehydrating water. There's your liquid base. The rest of the additions are up to you. (I had some leftover andouille, and so threw that into the pure as it did it, which thickened it and added a nice meaty element as it helped build heat.) I used it as a coating for a roast, but I don't see why it couldn't be -- in a more liquid form -- the base for a stew.