First I need to say: I love ISPConfig and this website because it (and ofcourse the forums) contains tons of information and it has helped me with problems that I could have never figured out myself. So 2 Falco: You're doing great work!
Here's my feature request that I would find very useful myself:
When I add a firewall rule and click save, I automatically return to the ISP Services page. When I'm adding or changing firewall rules I usually do more then one at the same time. So I would prefer that when I click save for a new firewall rule, that I return to the firewall configuration page. Not only to add more firewall rules but also to see that my new firewall rule is in the list.
In ISPConfig other pages and the tree menu also change back to default after saving certain things but they don't bother me as much as on the firewall page.
Just my 2 cents