I mean, why not put every conceivable thing into the game? There's no reason not to, right?
You say this sarcastically, I say this semi-seriously. Obviously I don't expect
everything to be included in DDN, but things like Second Wind are not the sort of thing that takes huge design space. You could make a pretty comprehensive optional rule with tons of the proverbial dials (i.e. per encounter, per day, per level, etc.) in about 3 paragraphs.
I get that design space is limited, even in a tabletop game, but I don't think it's quite as precious as people seem to think. This is especially true when it comes to optional rules that essentially already exist in other editions. Granted, such things need to be adapted for the particulars of 5e and maybe playtested a bit, but not to the degree that the core things should.
I like Second Wind (for some things, but not all) and said that it should be optional. If I want my game to have an extra-heroic vibe, with pivotal moments that things like Second Wind can help create, I'd use it. If I wanted my game to be ordinary schmoes in extraordinary circumstances, I wouldn't.
Even if I loved the mechanic with the most fervent of passions, I'd still say optional, because I don't wish to foist a non-essential mechanic -- that some dislike wholesale and some dislike circumstantially -- on anyone. By the same token, the no vote is, IMHO, kinda bogus. Saying no here just reminds me of the, I-don't-like-X-class-so-DDN-shouldn't-have-it threads and posts from back in June-ish.
If I could talk to the Devs and tell them what I want in DDN, I would say, I'd like it to have all the things please. And if you don't have the time/space/ability to include X, Y or Z, well, that's a bummer man, but I don't want the game to lack any class/module/option for lack of trying to get it in there.