Maybe Morrigan decides she needs to get into the Fade, or needs the mark for something else? Or maybe she just realizes she needs someone with a big stick to hit the Dread Wolf with, and the Inquisition is one giant stick. Who knows what she'll decide once she figures something out.
I wonder if the control aspect of the Well comes from Mythal's power or her divinity. If she were able to lock away/give away the latter, Solas might not have any control over what the Well does.
Morrigan has the ability to track down the cure for the Calling, or at least has a good idea about how to go about that, since that's how the HoF sets out on his journey if he romanced her. She says that in one of her conversations in the garden.
We know that Solas headed west, where the Wardens are, and where Hawke went, and I can't help but think that the Calling and the Warden taint have a lot to do with what Solas is up to.
Or maybe I just want to see Morrigan, Theron, and Mahanon track down Solas together. Add in Zevran and Fenris and I think my monitor would explode with the sheer amount of elven sexiness being displayed upon it.
I can dream, can't I?