It's interesting. For the size a little pricy considering many plants do not stay "miniature" forever. I Have been considering doing the same thing but putting a 3 sided extension (back and sides) over my aquarium.
And also you have to consider that many fish (Including bettas) are escapees, who will try to jump or crawl over the edge of a tank. Famous for this are killiefish. I also cannot see how you would incorporate the filters and breathing/cleanliness requirement needed in a tank. Where do you put your filter? Without a top, putting any kind of lizard or frog in there would be a problem too if you decided against fish. Even shrimp need a filter.
Hmmm. I am thinking the only thing you can put in there are plants.