https://raileasy.co.uk / https://trainsplit.com is the most flexible existing service I've found, but even that doesn't give you an "anywhere" option.

I'm looking at how to add price data to railraptor, but it might mean sacrificing the fully-offline capability... once I have prices it should absolutely be possible to build a filter along the lines of "find me the cheapest popular destinations that are at least 50 miles away".