I guess I don't see the "ancient intelligent species" angle in the Asimov sagas. I liked those series well enough, but they're not my favorites. In his universe, there's no aliens, which is a very odd thing in SF. It's all about relations between humans, robots, and technology.
Another in the vein of "ancient intelligent species" is Hogan's Gentle Giants trilogy. I'm not sure what you categorize as a thoughtful writers, but I would put him in that category.
Also, the basis of Saberhagen's Berserker Saga is ancient competing species who have destroyed each other with their creations - and the only thing left is their killing machines. These are purely xian propaganda, but pretty good stories anyway.
On a slight tangent, there's the Well of Souls stuff which is not great writing (particularly the second series which seems very rushed and poorly edited), but is an intruiging idea. In this story the universe as we know it gets reset periodically to start from scratch again. However, there's this one race that no longer exists, except that the guy whose responsibility it is to reset the universe who gets transmogriphied into a being from that ancient race, because they're the only ones intellectually capable of multitasking to the degree necessary.
I think the "ancient intelligent species" concept is pretty common in SF, though there tend to be 2 diametrically opposed views:
1) the ancient species have such perfect wisdom that they understand us thoroughly and we are mere idiots before them, we are unworthy to the vastness of their intellect. we simply cannot be trusted with their knowledge. Example: ST universe prime directive.
2) the ancient species are no better than we are, really, even if they think they are and we believe them. Examples: uplift saga and berserker saga.
There is a sort of one-off approach: Gentle Giants type approach. The ancient species is extremely wise, but they are unprepared for the visiousness of the human (and other Earth) species.
