With the basic shapes already created and with the easy-to-work-with nature of the materials, one could get quite elaborate with your fortress.
Consider this: If you have two demi-bastions, a bastion and two curtain walls, you have the makings of a crown-work. Two bastions and a curtain on their own make an acceptable horn-work, as well. These structures served to enclose detached suburbs, or extend the work deep into the countryside if needed.
Another feature that catches my eye would be to build a 'cavalier' or elevated battery inside a bastion, although I think I might have to re-build my bastions on a rather roomier basis.