The answer lies in the breakdown of how the organisation intends to hit it's goals.
See this answer for context - https://sharebird.com/h/product-management/q/how-do-you-handle-exec-input-in-the-roadmap-and-convey-a-point-of-view-while-also-accommodating-1
To take an extreme example, if 70% of the organisations revenue target is based on the acquisition of new customers then that's a good guideline for your investment levels, eg
65% - Prospects
25% - Existing customers
10% - Tech debt
I find that this type of division can be misleading. In many industries, there is little difference between what an existing customer would want vs what a prospect would want. Many times, the two would like to see the same kinds of capabilities built (but their priorities might be different). I think more about the buying centers I’m targeting, the use cases they have that my product can solve, and prioritize the roadmap based on the importance of those.
I wrote this post that covers a high level framework I use, aka Value stacking:
I will post a follow up on medium in which I'll deep dive on prioritisation, but there are many frameworks already in use out there, but the thing that I see missing is a coherent vision or strategy that provides direction that should inform your prioritisation.