How to best keep track of upcoming features-to-build?

We're very early stage and obviously, we could be working on hundreds of different things. We don't really want to create a roadmap, since every 1-2 months our priorities shift again. At the moment we're working with a Google Sheet that stack-ranks ~50 items we could do next. It feels very inflexible and manual. How are you dealing with this, any tools you could recommend?