Hey!!
I seriously wish there was a resource pack that would use blocks like diorite and granite (that people rarely use) in a way that they would indicate when interacting with redstone (Signal strength would change the block color it is on, and other stuff like that). I have no idea if it is possible or practical, but you asked for suggestions so I gave it anyway
Cheers! And good luck with your resource pack.

Look to things you enjoy for inspiration. I< for example, have always enjoyed cartoons, animation, comics, ect. and that really reflects in my texture work. Watch movies you enjoy, read your favorite book over again, find colors and ideas in places you'd never look otherwise and you'll get a really unique, fresh pack.

Before you texture anything, it's a great idea to assemble an overall palette for the pack. This will not only let you see how all the colors you'll be using fit together before you put hours and hours of work into molding them into the textures you want, but it further reinforces whatever themes or moods you want in place for the pack. I usually work with 16/24/32 color palettes, depending on the complexity of the pack.

Make sure to have fun with it! That was always my biggest obstacle with art in general. I wasn't enjoying projects. My first attempt at a publicly available resource pack, "Bitlets", became extremely tedious and frustrating for me, since I wasn't yet capable of dealing with the limitations I set myself. I kept grinding away at the project for a time, instead of just realizing that it wasn't meant to be and just moving on to a more enjoyable pack. This doesn't mean to quit at the first sign of struggle, it's very important to see through that, but if you're spending the great majority of your work time on the pack unhappy, I'd say it's time to move on to something new.

Hope this helps!

I say get inspired by the new LoTR game, Shadow of War for some blocks like maybe sandstone.