I just played a round of Machi Koro with a friend, and at a certain point, he was far behind. He didn't have many buildings that grant money on other people's turns, and the rest of us had also purchased a lot more red buildings than him.

I'm looking for ideas of house rules that make help provide some sort of catch-up mechanism if you're far behind. Do any of you have any tips?

If it matters, we usually play with the Harbor expansion, but are willing to play without it as well. We're already considering it because of how extreme the Tuna Boat can be.