Are questions about the in-universe lore or history surrounding LEGO sets or themes, especially non-licensed ones (ones not based on existing stories such as Star Wars, but invented wholly by LEGO) on-topic? For example, one might ask about the political structure of Heartlake City, how many planets Blacktron controlled at the height of its power, what the training regime is like for new Imperial Soldiers recruits in the Pirates universe, how one becomes an Ultra Agent, or the names of Jack Stone's parents. Answers might cite original advertisements, box blurbs, tie-in media such as books, or official staff interviews.
LEGO themes, factions, or sets that have clear science fiction or fantasy elements (e.g. Classic Space, Lego Elves, Ninjago, Exo-Force, Castle Dragon Masters, Time Cruisers, etc.) are obviously on-topic at Science Fiction and Fantasy.SE, but LEGO themes that have little to no SFF content such as Pirates, Friends, or Western wouldn't be allowed there. A brief review of LEGO-related questions on SFF.SE also shows that a large percentage of the "LEGO" questions there are actually about tie-in media such as films and shows rather than sets themselves, leading me to believe that set-based questions might not get good answers there.