Well I'd begin by adding another female molly, platy and guppy if you already have one male and one female as they should be kept at a ratio of at least 2 females to every male - if you have 2 males or 2 females, then don't worry about this. For the moment I would suggest some honey gouramies, sparkling or croaking gouramies or a pearl gourami.
Not all gouramies will get along but there are many options that would - amongst these are honeys which I highly reccomend if you've never kept gouramies before - they are small and quite peaceful. You could comfortably keep 5 with up to 2 being males and at least 3 being females. Make sure you don't confuse them with dwarfs. Honeys are colisa chuna/sota whilst dwarfs are colisa lalia.
Dwarfs are the next option. Though they are very attractive, I would not go for these as they tend to carry disease and you'd only have room for about 3 as the males are territorial. Unfortunately, females are rare to come by as they are not as colorful so don't sell quite as well.
Two larger but equaly peaceful colisa species are c. fasciata and c. labiosa - the banded/striped/indian/giant gourami and thick-lipped gourami respectively. You could keep about 2 males or a single male and 2 females of either (or a mix) without problems. Make sure you don't confuse the 'giant' gourami mentioned here with the real giant gourami (osphronemus goramy) as these get to 24"!
Pearl gouramies (trichogaster leeri) are my personal favourite. I have found these to be exceptionaly peaceful, considering they get to 6", and the most attractive. Mature males are absolutely stunning fish and are not as aggressive as many other gouramies towards each other or the females. I would reccomend a pair or trio of these for any tank of 20 gallons or more. They are also quite hardy and would get along with any of the previously mentioned species (though obviously you couldn't keep as many of each as I suggested as they all need some territory space - about 4 gouramies would be the maximum you could keep of any/a mixture of species). A pair of pearls could also work.
A single three-spot male (this includes opaline, blue, gold, cosby, platinum and lavender - all of which are color morphs of trichogaster trichopterus) OR a pair of females, could work but I'll warn you that these are aggressive fish and some can have a very bad temper (particularly males). They get slightly larger than pearls (which average 5"), at around 6". I would not mix these with any other top-dwelling species, especialy slow or vulnerable fish - that includes other gouramies and fish like hatchets. These gouramies are, however, the hardiest and easiest to breed.
Then there are sparkling and croaking gouramies which would work nicely if your tank is planted. You could keep quite a few of these (say 4-6) as they are not as territorial and don't grow big either. If you are lucky, you will even get to here them croak. They aren't as much of a top-dwelling fish as the previously mentioned species.
All these fish would work but would like floating plants or some kind of tall plants (near the surface) to hide in and to help devide territory. Be aware that the males are generaly more aggressive and keeping only one of the trichogsater species (males) is reccomended. The colisa species I mentioned would be fine with more than one male but choose carefuly and make sure any females always outnumber the males or they may be chased and stressed continuously.
I suggest you do some extra research on google.com and the fish index on this board for more specific details on each species. Don't rush into anything and ask if in doubt. Keep in mind that gouramies can be temperamental and their characters vary widely so you may need to switch between individuals before getting a compatible group.
Also visit your LFS and see what's available.
Do NOT get chocolates, licorice, frail, osphronemus/giant (not the colisa species - the real one), kissing or moonlight gouramies as they either get too big or are very very fragile. Moonlights actualy don't get THAT big but I still would not reccomend them if you want more gourami species as they need space - their aggression level is somewhere between pearls and three-spots.
In each case, I have not realy thought much about how much space you have left in your tank for additional fish so please make sure you don't over-stock. Keep your nitrAtes in check and keep up with maintainance to make sure you don't put in too many fish and cause a fluctuation in water parameters or end up with bad water quality as your fish will not like this and many gouramies are quite sensitive to such changes. There's no need to worry about pH or hardness (unless your's is extreme) as the species I've suggested are quite adaptable.