You are asking about what appears to be a total waste of space. And what to put in there. If you are happy with the filters you have and the one heater then yes it’s a no fish waste of space, with a filter you can’t see and a heater you can’t see to check on.
I have several second hand Fluval tanks . I got stuck into them straight away and cut the black silicone away the removed the panels. I no longer have to hunt to see what of the life support system is working.
So my answer is fish now swim in that wasted space I can see if a heater is working and can check the status of the filter at a glance.
My opinion filters and heaters should be checked daily not hidden away like they are something to be ashamed of.