definitely a sweet/savory jelly would be good, jalapeno, or green chile
or red chile (available online definitely or at specialty stores)

for a simpler filling, you could use some brown sugar and a little
almont extract. they roll slightly damp grapes in powdered sugar or
bakers' sugar crystals.

also a good ideas for grapes is to go savory instead of sweet, and mix
cream chesse with shredded, good quality sharp cheeses. a sharp cheddar
goes nicely with grapes. mix in a little roasted red pepper perhaps. a
good emmanthaler or jarlsberg would work too, with a touch of mustard
seed.