The Ro unit will mean u be able to experiment and get the desired water type removing most restrictions :) oh that tank is so pretty if I was more experience I would go marine but so not ready to make that transition as tempting as it is lol
Gosh that's a tough one - so many ideas for a tank that size I don't know where to start!
An upgrade to a similar sized tank is on the cards in a year or two anyway, but with it being largely for Hermione's benefit it will have to suit her. Probably add in some Clown Loaches, and move my Severums and Angels in there along with a large group of bigger Tetras - maybe Colombian Tetras. Sand bottom, large smooth boulders, bogwood and only tough plants - anubias, java fern, plus plenty of easily replaceable elodea
If I could have two the second would be a group of Discus, Sterbai Cories and Cardinals. Sand again, chunks of slate and some small cobbles, lots of twisty roots and heavily planted.
If I can have a third (and a bigger house) - a massive Tanganyika community would be fun. Large rocks at the back built with overhangs and caves, sand substrate, lots of shells for shelldwellers and maybe tough plants again lodged in the rock work, plus some Vallis.