You garden is your world, plant whatever you like. My suggestion is to plant in clusters or odd numbered groups like 3 of this, 5 of that, 7 of those. Even numbers turn out to be more static, less effective.
My three personal favorites are astilbe, hosta and coneflower. But I enjoy seeing iris, daylilies, primrose as well as dozens more.
Also, take it slow. Watch how you plant things, taller towards the back, shorter up front and arrange your planting's so they flow with gentle curves and rounded edges. This I find is more pleasing to the 👁, the garden flows better that way.
If you have a real sunny spot consider a Little Rock garden with hens and chicks, Sempervivums and others. You can keep adding both rocks and plants as your garden grows over time. These guys are so cool just for the shapes and textures that they bring to the garden!