I really like that tank! I change mine a lot. I think you could keep doing tanks with your current theme, maybe use different decor items and add more live plants or even try to make a moss wall? I want to make a moss wall myself but I've been busy with making other DIY things at the moment.
Maybe you need just 1 little hidey-hole or cave for your bettas? But I really do like your theme, it isn't tacky, it's clean, and I like white with only green of plants adding color. (I have made my tanks too tacky and then taken everything out!) Keeping the tank the way you do allows the fish to be the only focus as far as any color standing out.
The only other thing I like is sometimes adding a small object of a similar color to your fish- like just one red thing if your fish is red. . . or I use a few ceramic beads that bring out my fish's color just throw into the substrate.
My tanks are in progress but I can show a photo- just keep in mind I am still working on them to make them less tacky.
That is Alejandro with one gemstone rock I found that matches him, and a couple ceramic beads. He has no substrate now because I am going on vacation and want to be able to clean easier and show my mom how to suction up food & poop while I'm gone.
If you look through my photo albums you can see how I tried to match some colors of my fish to their decor .. . . but I am also trying to take out a lot of things and simplify now. It's a work in progress!
Here is Cornelius- he's white, purple-pink, with some red-pink on him, so I try to use those colors. Also .. now I made his background black and I am gluing black sand to some caves with aquarium sealant, since a white fish will show up better next to back sand and black background. His tank will still have "hits" of purple-pink, white, and hot pink. (I am a clothing graphic designer and textile designer.. . that's why I'm obsessed with fixing the color ratios .. ha ha)