Ask Rami: Don't roll the credits just yet.

"We are developing a roguelike, and we've just done our first semi-public playtest. A lot of players are finishing all six levels on their first few attempts and complaining it's too easy. We didn't expect this, we don't have a lot of money left, and we really need to stick to our release date, so we can only add a few new levels. What can we do?"

Hi, roguelike developer!

First of all, I'm sorry to hear about your situation, and can imagine it causes a lot of stress. I don't want to rub salt into a wound, but it would be remiss not to start by saying that I hope this shows the importance of playtesting much, much earlier in the process. I don't know how long you've been developing, but if you're running out of money I'd assume you've been going for at least a year. This honestly is way too late to be realizing your game has such a fundamental issue - and the best way to prevent this on your future projects is to ensure you playtest earlier.

So, without knowing anything about the game, it might surprise you that my answer is not necessarily "make it harder". My answer is "do not roll the credits, not yet".