As for beautiful it's subject so sorry if it does not live up to what you are looking for. As far as beautiful games that can be emulated via RetroPie I can think of a few.
You could also look into RPG Maker games such as "Lavender", At least the titles that you can emulate for RPG Maker 2000/2003.

"Super Mario World 2: Yoshi's Island"has this beautiful pastel color and very vibrant look and feel to it, really good for the time it was released.

"Summon Night Swordcraft Story 2" had some nice visuals for a GBA game (Action JRPG)

Summon Night - Swordcraft Story 2 (USA)-211105-114835.png