Nothing they can swallow or get lodged in their throat, so cories/otos etc are out.
Nothing that can latch onto them and suck their slime coating, so plecos and the like are out.
Nothing that likes tropical temperatures, so most everything else is out.
Most serious Goldfish fanciers keep their Goldies alone- with other Goldies.
Possibly a Hillstream Loach, but again a curious Goldfish can suck them up pretty quickly, and if you haven't got long tweezers and a steady hand you can lose the Goldfish.
Two Goldfish in a 35 gallon is very close to being maxed out anyway.
Sorry!