When I was making a vegetarian version of a shepherd's pie a couple of weeks ago, I found I hadn't made enough potato to give a decent layer on top of the other stuff that goes in to one. I've had this happen in the past a couple of times, and fixed the problem by adding carrot to the mashed potatoes, with good results. Except, I didn't have enough carrot either. What I did have, however, was a can of pumpkin puree. So I decided to use that to bulk up the mash for the topping. A bit of advice if you're thinking of doing the same thing... Don't! Adding pumpkin puree to mashed potatoes in general works. It makes a lovely creamy mash. But it didn't seem to work too well for the shepherd's pie. It wasn't dreadful, and I ate it. But it made the shepherd's pie taste wrong; it was like it wasn't cooked properly, even though that wasn't the case. Honestly, I don't feel that particular cooking experiment was a success, and it's one I certainly will not be repeating.
If you'd like some recipes that are worth trying, however, check out my website's recipe section, where you'll find all sorts of recipes. Most of the recipes on my website are suitable for vegetarians, and some are even suitable for vegans.