Problem with removing time limit on instanced dungeons is mainly some parties would just stay inside all day, not allowing other ppl to enter, hence also the timed re-entry. If they weren't instanced and therefore not timed, you would also have people camping inside all day. Maybe once we have new servers, these would allow for a longer time limit and even a wider allowance of parties until server congestion, so indeed puzzles and the likes have a chance to be implemented. The idea of course is nice, but I don't see it happening anytime soon.