Can you frame a pretty stamp? That incorporates some of these colours or use a frame that echoes it... (It would need to be smaller than the lower right and possibly another square)

May I make a suggestion? You should move the piece in the lower right corner just a tiny bit, so the spacing around each frame is consistent.

Some stunning pieces :) Definitely make sure that all the frames are the same width apart even if they're small, it will allow you to add pictures to the gallery in the future and also make sure that you see the pieces individually.
Colour wise- blue or pink but incorporating the other colour (just as the other art does)

Blossoms

I think instead of adding more art I'd suggest you find a trio of miniature white iron accent pieces or framed small mirrors. Your art pieces are beautiful and you don't want to take away from them. Top left..yes...scooch your bottom right pic over either way yo put another piece and possibly reserve the last a cent to allow you to expand your art over time. I can't see your snapshot while I'm typing this with the Houz app so I'll likely have look again to give any further suggestion.