I know what you mean by too symmetrical , that's what I'm working on now, I saw a site member had what looked like lava 'caves' and I was thinking of building them in the back left then some short grass for the right hand side.
And ATM I have
12 x harlequin rasboras
4 x albino cories (hoping to raise this to 6/8)
1 x red betta
I don't think that's too bad so far but I don't think I'll get much more in the 60l. What do you think?