Discussions about interest aside, I would agree that another main quest story would not be in your best interest if you are seeking meaningful critique. You said you had something in mind for before your protagonist is thrown in prison, why not expand upon that? I wrote a whole story to set up a completely different plot line about two years ago, and I ended up liking it more than the actual story
To sum up what just about everyone here has said, do not write what we want, write what you want. If you want to sell a book, write with an audience in mind that will love your story; if you want to have fun with the book, write whatever comes to mind. Don't make it some absurd tale about Guar and Nirnroot, but pick something in Tes that interests you and go with it. When I started out I loved assassins to death (pun intended), and that was all I wrote. After that I moved into necromancy, arena-fighters, the Dunmer-Argonian slave trade, and even skooma based stories; my point is, write about anything, but make sure it interests you.
If you're starved for ideas, you can take a look at some of the lore- that usually inspires me to think of a "What if?" scenario. Or, if you're not as much into lore (which I do not recommend), think of a book or movie you really liked and base a story off that main idea.
Either way, the good thing is that you write something
Keep up the good work!