I've been thinking a lot about this. Since switching over to a system of styrofoam hills with game mats on top, it has opened up lots of opportunities to try different things easy and cheaply. It also got me thinking about how much actual contours vary in real terrain.
So I am starting to try different thickness of hills on the same table. I have a whole bunch of cut out and nicely contoured hills sanded smooth (but still with flat tops I can stack). Now I am thinking about going the route of thicker AND thinner foam. I found a bunch of 1/4" thick foam I want to try using as well to make some very subtle undulations in the ground. These will be sanded with a long slope and smooth so that when under the mat they will be a subtle variation in terrain. I also want to try some 2" thick ones as well to make more severe changes.
In experimenting with this I also discovered a way of doing changes in terrain other than just simple hills and elevations
take the same concept, and use it for raised roads, elevated fields etc.
I have done this for roads and it worked well. By using a felt road placed on top of the mat and lining it with trees it made for a nice road effect for places like Holland and such. Also, if you use foam squares for fields it raises them up so that around the fields, you end up with a nice ditch effect (especially if you place hedges around the field perimeter and leave space between the field and the hedgeline).
Also, by using thin foam, you might be able to get those subtle dips and folds in otherwise flat terrain that is so hard to get on a game table, but so good at concealing infantry on a real battlefield.
If you reverse the concept, cover the table in 1" foam and cut down into it where you want rivers, lay your mat down and plunk your pre-made flat rivers in the depression
add scatter rock and bushes along the banks and easy instant realistic rivers (as well as ditches etc.).
My plan is to create a whole bunch of 2'x2' foam tiles to lay under the table. Wherever I need a river or depression, I cut a special one out that fits into the tile system. Then I can add hills and stuff on top of that to create much more variety in changes in elevation, all by just cutting out foam. Since it is all covered by a mat, the work is super fast and easy. It all breaks down for storage nicely as well.
So far, I have only experimented with a little of this. It took the release of those GW game mats that follow shaped contours so nicely for me to realize it could be done.
I say go for it and share your results. I'd be interested to see how it worked for you to get some further ideas.