I really like this, if they Valve expand on medieval mode and give it some more maps (and for some reason I can't seem to find any custom medieval mode maps) they really need to give the engineer some sort of siege weapons. The trick is making them so that it makes the engineer competent, but not to the point that it breaks the entire mode. I mean, he is an engineer...
Would have to be something that's not essential for the map, though. Class synergy is great, but it's actually an immensely important part of TF2 that there's no specific objective that only one class can do. I've seen a couple of maps run custom medieval maps, but while some of them are nice and big compared to DeGroot Keep it's not exactly the same standard.