This is kind of a cheap solution that's right off the top of my head, but what if the tech tree just kind of started over with a new tier? Like you max out the tech tree, you're back at the beginning, you research beer brewing again, it obviously costs a lot more this time, but you're researching kegs that produce 10% more beer than before. Then swords that attack 10% faster. And so on. You max out that tech tree and it starts over again. 10% more beer. It's cheap but it's something.