Replacement for pumpkin
The closest in terms of taste an texture would be another winter squash such as butternut, acorn, hubbard, etc. (As has already been mentioned.)

However, anything with a similar texture would work. If you don't care so much about the taste, you could substitute a tuber such as potato, turnip, rutabaga, carrot, parsnip, celery root, or parsley root. 

Sweet potatoes or yams would be closer in taste, but might be mushier than you want. 

I'd suggest experimenting with any single or combination of the above and see what flavor you like best.