So, actually, it's not about "potential markets", it's about competition for existing markets?
I don't know who the people from Newark and Lincoln you've spoken to are, but I have family in Lincoln and they have always found the service into London from North Gate brilliant, and frequent enough to meet their needs. If you want an improvement in comfort, then don't try Lumo (basically the same train). If you want an improvement in pricing, then, frankly, tough - quibbling over £13 for a journey that's around the £100 mark is hardly something to base a reasonable open access operator on. If you want an improvement in reliability, well, avoid Grand Central.
This leaves Hull Trains as the only possible option, or a new OAO that, as has been discussed previously on the thread, wouldn't get off the ground. Now, in terms of potential markets that aren't or might be (could you clarify what you've meant by potential markets please, as it's definitely confused me?!) Hull could potentially be viable, but it wouldn't be. Newark North Gate is, as I've said, essentially a parkway for much of the surrounding area, and a large chunk of its custom is Lincoln. From Lincoln, it's already possible to, which much less fuss than stopping an HT train additionally at Newark, drive or take the train to Retford and pick it up there. So an HT stopping at Newark North Gate would only really tap into almost exactly the same market as it does at Retford.
Again, I don't know whether your Lincoln correspondents have a different definition of reliability to mine, but the perception I gain from them is that the existing LNER service is frequent, reliable, and trustworthy, with all the possible places they would like to go to northbound and southbound covered easily, even though some require a change. This is not a position that any of the other stations along this section were in before OAO came along, except for Peterborough, which, if I recall correctly, is only being allowed because the GC services in question are looped there anyway. They pass North Gate at 125mph.
If it ain't broke, don't fix it.