Based on the quote and prior comments from the dev team, my guess is that it would be FE6. It seems much more likely to me that FE6 would be remade without FE7 than FE4 without FE5, and based on the admittedly slim precedent of FE11/12, I don't think they'd split a pair of remakes across different systems.
Are we thinking of the same game? FE6's gaiden requirements are all to play fast and keep certain units alive, and then don't break the Divine weapons. YMMV I suppose, but to me that seems like something players would naturally do eventually. The only one that's really obscure is 16x, but that could easily be hinted at by a base conversation.