"-Allowing Link to travel underwater again" - YES
And a day/night cycle that works according to real life, including a season system. If I turn on my console at 9p.m then the game should be presented at night too.
Only if that feature is optional.
When I only have time to play in the evening due to work I don't want to move through a dark sunless Hyrule every evening. Shorter day/night cycles in games are more interesting IMHO.