Depends what you want. Some people want aquaria to be low maintenance, in which case a mature, stable aquarium is what they need. For those folks, sitting back and enjoying the view is all that matters.
If you're a fiddler, then replacing easy plants with more difficult ones is one way forward. This could entail increasing the lighting, adding a CO2 generator, or even replacing the substrate entirely.
If you're an advanced aquarist, then rehoming fish you already have might be an option, so you could make space for more challenging species. For example, you could replace tank-bred angelfish with discus, or even wild-caught angels if you had the skills.
If you're anxious to try something new, then you could replace some of the fish with others so that you create a specific theme, perhaps an Amazonian blackwater stream or a Southeast Asian brackish water creek. Often people find they like specific fish best, so creating a tank around that particular fish makes sense, while removing any species that don't work in that context.
Breeding fish is, without question, the ultimate test of your fishkeeping skills. It's an objective measurement of how well you can keep fish because only under the right conditions will fish breed. Admittedly, some species are easier than others, but even so-called easy fish like guppies are more difficult to breed *properly* that many people realise. Simply cranking out hybrid mbuna or crossbreed guppies doesn't take any skill at all, but carefully producing quality fry that retailers will want to buy is a bit more difficult.
Cheers, Neale
I am getting to the point where all my tanks are stocked the way i like it...
I was thinking what the hell am i going to do next...