I use whatever I have on hand... (most of the time I season with garlic powder and onions or onion powder.)
We've had turkey shepherd's pie with leftover turkey, gravy, and potatoes from Thanksgiving or Christmas. Veggies were whatever I had in the freezer.
I've used leftover roast, with gravy, mashed potatoes, and whatever veggies were in the freezer.
And, ground beef, cream of mushroom soup, mashed potatoes, and whatever veggies were in the freezer.
Obviously I'm not too picky about what veggies I use :P I do think it's weird if there is only one kind--just corn, just carrots, whatever, so I usually do a mix. I almost always have a mix of peas, carrots and corn in the freezer. To make it more "odd" is the fact that I hate vegetables... But, I figure this is a good way to smother them in gravy.
To me, it's one of those foods that doesn't need a recipe or to always be the same--just dump whatever sounds good that day into the dish. It's also a great way to use up leftovers. I figure if you see it in a pot pie, you can put it in a shepherd's pie.
One time, we had a refrigerated pie crust (before I started only making my own) that we needed to get used. So, we made the pie in that--so half "pot pie" half "shepherd's pie".