How do you test new features/ideas when you don't have traffic?

That's a real challenge. Can you get some BETA users to test your product for free and then ask for their opinion, or even better, so how they're actually using this new feature?

I try to put myself in the user's shoes and just see if I like it / use it Also, a good way is to share it with people you know and ask what they would add / change (don't ask for feedback, they won't give you honest replies because they will be afraid to hurt your feelings)

I think it was already mentioned, but finding testers through friends and family can be the most solid option. If you have a bit of spending money, try running some ads to get users to come to your app. Additionally, consider if there are any online forums or places where you can freely post your app in a relevant location where users will be interested to test something out. Maybe a certain subreddit.