Changing the excessively huge (and probably financially needed) capital gains deduction for selling a home and buying a new one to only work if you literally buy new construction - would probably spur something.

Existing stock would be depressed in value as the pool of buyers would be smaller, but they'd be more likely to be first time buyers anyway (no capital gains to deal with) and demand for new would increase.

But you could end up with insane inversions where existing stock was pulled down to build new to take advantage of the gains exclusion.