Since the wall's faces are going to be sloped (somewhat like the trace italienne that formed the core of 18th century Europian warfare, or earth mounds of the Spring and Autumn Period of China), why not follow the examples of our RW ancestors and top them with a smaller, straighter wall backed by a road? By nature sloped mound-walls are not designed to block passage and rather are elevated firing positions. Well, higher inclines do prevent perpendicular movement, but in DF you can't control the slope angle, so for the structure to actually function as a wall it needs to be topped by a wall.
You can also place catapults and ballistae on bastions, which I trust you already know of but in the slight chance you haven't heard of them, an angular formation jutting out from the linear mound which serves the same role as towers on castle walls. Of course, in DF siege artillery only works on the same z-level, so they are only aesthetic additions, but they'll still look cool.
The walls of Constaninople which were famous for their nigh impregnable defensibility were comprised of double walls, so that the inner, higher wall provided additional firing positions and doubled as a backup defense in case the first wall was taken by invaders. In DF the enemies never scale walls (
yet, Toady says he plans for advanced siege warfare in future versions) so double walls are a redundancy, but when completed will look pretty and nasty.
Now, enough with RW ideas. On to the dorfy schemes.
A water reservoir placed atop the slope can wash invaders downwards into whatever fates you deem fit. A prime option would be a pit dug right outside the mound-wall with upright spikes planted in them. Or, for bonus effects, replace the water with magma and you can send your pesky besiegers burning whilst rolling down towards their doom. For this to work as intended you need a method to lure the hostiles all the way to the top of the incline while not giving them a chance to cross it... or the whole thing would simply be open and designed symmetrically so when magma is release in the right moment the enemies are swept in two directions which both feed into pits. I know this is much more complicated and also less defensible than the other options above but it's way more dorfier.
Also, a drawbridge that opens outwards can be place on top of the mound. When activated at the right timing it will fling enemies outwards, so they experience the bliss of free flight a few moments before falling down stunned, or if you're lucky, dead. Beware as building destroyers will simply destroy the contraption on sight.
Limitless possibilities still remain, but the above are what I could come up with in the short time. I hope I was of help