I started collecting Lego because of the Hobbit and Lord of the Rings themes and now because they are discontinued, I seem to have the same problem. (I'd show more interest in the Star Wars themes if there were more sets out there which were related to the Classic Era but most of the ones out now are related to the new films (which I'm not interested in) and the Classic sets which are around are too expensive, so maybe.)
I'd like there to be more fantasy/historical themed sets which aren't eye popping in their colours because these would be good additions to my Hobbit/LOTR themed sets.
But I think the main problem of why sets don't appeal to me is the small number of pieces in them and leaning towards smaller models, but the prices remaining high. I did notice in the later Hobbit sets in particular that the buildings got smaller (mainly because the pieces were smaller...), but the prices were higher. If you compare Barrel Escape to the Mirkwood and Lake Town, BOFA sets, Barrel Escape is quite a large model and wasn't that expensive, but the models in the other sets were tiny. (They were still fun to build, but they would have been better had they been bigger.)
When the Simpsons theme came out, I was excited for that as I'm a fan of them and looked forward to collecting them, but I don't understand why lego stopped releasing the buildings and figures for them. (I like the Simpsons not only because I'm a fan of the cartoon, but also because I like how the colour scheme has been incorporated - sometimes I don't like the CIty and Creator sets because the colour schemes are a bit dull, but sometimes the way the Elves and Friends sets are put together, the colours are a bit eye popping.)
There is a good range, but I'd like to see the return of the themes which I missed or enjoyed building (or if not returned, at least have something similar to them.)