Save images to atlases. Using something like TexturePacker. Reduces draw calls thus improving performance.
Use responsive UI design from the day 1. Make graphics and UI supports all the major resolutions without breaking. It's a lot harder to add multi-resolution support later.
Simplify lightning effects. Use baked or semi-transparent PNG image. Consider removing or faking shadows. Improves performance.
Support multiple input styles. Try designing game mechanics that support multiple input styles. Mouse, touch, only keyboard etc.
Iterate fast. Start with something that you know so technology isn't a blocker.