If you wanted to play about with the flavours you could always make the filling mint by using mint extract and fresh mint leaves. Personally I don’t like the taste of fresh mint leaves in sweet dishes so I would use mint extract and a few leaves of spinach just to give a bit of green colour (about 3-4 leaves).
Or you could leave out the orange all together have them as chocolate coconut brownie bars. If you wanted to enhance the flavour you could use natural coconut extract.
Other flavours to try would be raspberry (swap the orange juice for a handful of fresh berries and natural extract), vanilla (leave out the orange and use 1 tbsp of vanilla), or chocolate (leave out the orange and add 2 TBSP of cacao powder and a few extra drops of stevia).