Here's the thing:
5E isn't missing much really, IMO.
The issues are just that the rules that are there seem incomplete or are too simplified.
For example of an incomplete rule (or system), you can certainly grapple someone, but how do you restrain someone? It is possible in real life to restrain someone, so how do you do it in 5E?
Apparently the only way is with the Grappler feat. But feats are optional, so if you don't use feats it isn't possible.
And what do the designers say? "Just rule it however you want."
Why? You have rules for how to grapple, why not how to restrain? Would it be so hard to include another rule for restraining?
For an example of over simplified look at the rules for jumping. Up to your Strength score, with just a 10-foot approach. Pretty silly, really, and too simplified. Under Athletics, they even specify making a Strength (Athletics) check is done when "
You try to jump an unusually long distance..." But again, now we have incomplete rules. What is the DC? How much further can you jump if you make the check? It is always just left up to the individual DM.
I know you can't make a game where you have rules for
everything, but frankly my biggest issue with 5E is it seem half-assed.
Of course, some systems
are missing, such as others have mentioned, like dominion development or mass combat, but personally those aren't big things for me. I'd rather see concrete, complete rules for the systems we have, first.