You'd still need to solve the double-spend problem, because while contract state on Freenet will usually synchronize within a few seconds, it isn't guaranteed to converge to a single globally consistent state.

Freenet's approach works well for things like group chat, where temporary inconsistency is mostly just an irritation, but for a cryptocurrency it is fatal.

I'm not saying you couldn't build a cryptocurrency on Freenet, but you'd still need a solution to that problem.

Ghostkeys seem like part of a decent approach to this, since it's a "prove contribution without dictating how" system. Crypto/cash/"they gave me a high five, they're cool" are all equally valid, and it's not a proof of work that costs substantial money to operate: https://freenet.org/ghostkey/

Currently it appears centralized, but in principle it'd be pretty easy to shift it to a web of trust instead, and hosts can choose what they allow and how much they value it.

(zero-knowledge proofs seem probably rather important to adopt tho, as right now it'll tie you to a stable pseudonym)

Cryptocurrencies have a toxic reputation. Associating one to this project is a sure path to killing it.