Another rabbit! Rabbits and Guinea pigs shouldn’t be kept together.
From experience, I'd say the opposite. Ages ago, we had a single rabbit and a single guinea pig happily living together, never a bit of trouble.
After a period of a few years without pets, we bought a rabbit who was 100% soft and docile, but over time, we we felt sorry for her, so got her another rabbit. Despite introducing them over a few days (i.e. completely separate, then in separate cages but put together so only the grill was separating them and no obvious issue, it was hate at first sight when they were first put together and I've never seen such a fight - hair flying etc. Eventually, after many, many days of gently introducing them (sitting them together and stroking each together), they learned to tolerate eachother, until the day of the grudge match, when they had the fight of the century and both ended up badly bleeding!
We kept them separate after that so we ended up with two lonely bunnies. Until, that is, we bought two more guinea pigs, one for each, and there'll all living happily now - all 4 of them, but in two separate pairs/cages.