I absolutely LOVE this idea! Esp the skill challenge in the middle idea.
Solos are indeed too static, as in the solo stands there hardly moves.
I love the idea of having my players tracking a dragon into a cave, fighting it, it blasts the cieling, and takes off. Now the players hop on thier mounts, force it back to the ground in completely new terrain via skill challenge, allowing for awesome cinematic creativity, then a final fight at the edge of a waterfall or lava flow or something epic.
The idea of the lich, fight a medium sized lich, he realizes he underestimated the part, throws up an impentreable wall of shadows type thing and goes into a cacoon of necromantic energy while he focuses, summoning an enouncter of zombies and skeletons. While drawing in the necromantic energy, he provides a leader bonus in the form of say, an Aura 5. Any ally of his in the aura gains regen x, and enemies regain only half of any hitpoints they would normally get from healing. After the skeletons go down, he breaks his shell, doing some damage, and grows to large or even huge size for the final fight.
That sounds like a load of fun to me!