Any European cyprinids should work, including common carp, tench, orfe/ide, bitterling, chub, roach, dace, common minnow, etc. Sticklebacks would probably also do well.
As far as trout, there are two native species in Iceland: brown trout (Salmo trutta) and Arctic char (Salvelinus alpinus). Either should do well in your pond, though the brown trout get so big you would probably have room for only two or three. You could probably keep a breeding population of char.
If you can get them, North American or east Asian temperate cyprinids, suckers, loaches, or catfish would also work. Just be sure to check legality, and don't stock exotics in your pond if it is connected to any other body of water (even during 100-year-floods).