Macaroons are sweet so filling can be fruity or chocolate or just simple cream. Here are some to try. Always be gentle with macrons when filling them.
Vanilla Buttercream recipe: I made this and substituted (double) orange extract in place of the vanilla. Awesome!
Mermeladas de macarons
This smooth, creamy mixture of lemon, eggs, sugar, and butter has a refreshingly bright, tart flavor.
Chocolate Ganache Macaron Filling: Heat cream almost to boiling point. Add chocolate and let sit. Whisk until chocolate is melted and smooth. Let cool before piping onto the macarons.
Blueberry and Lime Macarons: how I mastered macarons with a few tips and cheats. Filled with a quick, home made, blueberry and lime curd so they burst with flavour.
Raspberry Macaron Filling. I like the idea of combining jam with the buttercream.
Pistachio Buttercream Frosting Recipe for macaron filling | Bakepedia
The Tough Cookie | Salted Caramel Macarons With a Whipped Caramel Filling | http://thetoughcookie.com