Regarding sandboxes: The genre on it's own is not having less potential, but has an unlucky history in here. If you make a cool and good sandbox, that will get by time good reception, however predecessors used it in no-brainer way with no sense to gameplay experience - which has made sandboxes kind of unwished for.
I can imagine a good sandbox like old Frank's Adventure series. Of course that was meant to be short game.
Also, big part of the players do like to find all content, and sandbox poses the possibility that some will not be achievable in one playthrough. You can employ smart ideas to prevent the case this case, as mentioned previously.