I agree you have space, but before looking for different species, I would recommend you up some of the present species. Shoaling fish need groups, and while six is often suggested as the minimum, there is no doubt that more will always be in the best interest of the fish. And you have a couple species which definitely will be less stressed with more of them, namely the rummy nose tetra and rasbora (which I assume is the popular Harlequin, Trigonostigma heteromorpha.
A group of 12+ for the rummys, and with your 180 litre/50 gallon tank, I would go with 15. The Harlequin Rasbora I would increase to 9-12.
The Zebra danio should be fine, but a couple more wouldn't hurt. Corys too are better with more, another 3-6 perhaps. The pleco is fine alone.
With respect to a "feature" fish...I'll leave that until we know your choices. I'm not inclined to "feature" or centrepiece fish, as I find they don't always suit tanks of shoaling fish. But it is workable, if space suits the intended fish.
Byron.
