"I'm doing open-ended language design recently, without a single target language in mind. But I'd say I haven't considered any target language that would be up to this task!"
Actually, I'm a bit more hopeful than that now.
Hackability without breaking abstractions: One post can define a module, and another post can come along and define a different module in terms of the first one's implementation syntax. Maybe a module could do an import but then delete certain declarations by pattern-match and insert some new ones. Some declaration semantics could auto-erase earlier declarations, which is the same kind of feature that could be useful for REPL experimentation.
Fuzzy, living dependency resolution: Modules within a forum thread could refer to other modules in that thread by a simple tag system, which people could add and remove tags to after the fact. Underneath a unit test module, you could see a checklist of the implementations that satisfied it, and underneath any other module you could see a checklist of the unit tests it passed. Any post in the forum could refer to modules from other threads by using a slightly more cumbersome syntax, and this would be automatically cross-referenced at the original location.