What do you mean by "high concept"? You've mentioned it a few times, and it doesn't seem to match up with what I can find on the subject elsewhere.
In general writing terms, the phrase “high concept” just means a piece of media – typically a film – whose premise can be summed up as a pithy what-if statement. Like “what if a dinosaur theme park went off the rails?”, or “what if being an exorcist was like being an exterminator?”.
In the context of indie games in particular – meaning both video games and tabletop RPGs, to varying degrees -- the phrase “high concept” tends to have the additional connotation of taking such a premise and carrying it forward to its logical-yet-utterly-batshit conclusion, especially when your starting point is either deceptively straightfoward or very abstract. You know, stuff like “what if the player character in one of those Choices Matter™ RPGs were obliged to engage with moral risk on the same level as the actual player?”.
