I think you could go without a hardscape, as long as you were thoughtful about positioning of the different plants. Personally, I think what's most important in making tanks look good is a sense of depth and layers. You can landscape with the plants, think of it like you might think of a flower garden. Put taller plants in the back, shorter in the front, but to really make up for a lack of hardscape you'll want to make sure you have some midsize plants in there for the middle (or keep plants that would otherwise grow tall well pruned in that area). You can experiment with different layouts, using the same kind of rules that you would for arranging a hardscape, but relying on plants instead. The best thing is, the plants will grow back if you decide you don't like how you pruned them. It might also be good to rely on color contrast for interest. That's pretty much in line with the Dutch style aquariums, but I think you could also do something more amano-ish in appearance with the right contrast of sizes/colors/textures.