Depending on who you are talking to people reccomend that you have 1 discus to every 40-60L of water, and you have more than 3, or a pair, more discus also means more work (W/C's ect)
If I were you I would leave it with the two discus if they are happy
If they want some kind of plants we will need to know about the lighting/substrate first
good/easy plants are are things like anubias, most crypts, java/willow moss, java ferns and quite a few stem plants
Fish wise, larger single species schools of things like tetras/rasboras look good, Cardinals would be a good option for the higher tempratures of the discus
and then a few bottom dwellers: BN's/plecos, corys, and loaches would be some options
and as SamH always says:
HTH