The issue here is that I have ideas, but they would all be too problematic.
See, I love Merlin but its too historically innacurate to make a good Doctor Who episode. Also, Arthurian legend was already sorta-kinda tackled in Battlefield. Casting issues with Colin Morgan, Anthony Head and Richard Wilson as well.
Another one of my favorite shows is Downton Abbey. Yes, it is historic, but I also think its the type of show that would suffer if it had a run-in with a Time Lord. Also, casting problems as well. All British television shows seem to run into each other with that.
I also enjoyed, for its short run, BBC’s Robin Hood. Again, there is severe problems with historical accuracy, and minor casting problems. It’s also over, so there is that problem.
Just about the only other shows I watch are Bones and NCIS occasionally. Both American crime shows, which would clash horribly with the plot of Doctor Who. The general idea is, though, that any crossovers I would like would be incredibly problematic at best.
Just wanted to point out that Angel Coulby was in an episode, too (The Girl in the Fireplace).