It's not impossible to make it, maybe based on something similar like Tor, but everyone would probably have to download everything, and I am not sure many people would do that.
I expect something similar to how Bitcoin is used: some people let Bitcoin Core download the full blockchain, but the majority uses light wallets that rely on a centralized server. Or multiple centralized servers, such as Electrum is using.
When a decentralized forum gets too big, we'll end up with centralized servers again.