Either Jeb or a ‘moderator’ will throw out the old ‘how do you deport millions of people’ line. Hopefully Trump will point out that once we strictly enforce e-verify and make social welfare benefits less attractive a huge number of illegals will ‘self-deport’.
One question that might stump him would be,
"If you lower taxes, build the military, and balance the budget, the only way will be to cut non-military government social programs probably including entitlements. Which ones would you take away?"