Well I only want to assign this ticket as a 2nd and more precise part of this ticket:
http://feedback.arma3.com/view.php?id=2714
cos still I don't see any melee upcoming plans.

Well it's not much to say, every soldier have it's own hands... and still deadly knife used in survival and self defense.

Imagine you are out of ammo then what? currently you are unable to knockout enemy with hands or stealth kill him with knife (on logic if you know you can, why use gun and risk being spotted?)

I have a proposition when you could use melee:

1. no weapon/0 ammo - hands

on stage when you have no weapon cos you was cough as a prison and trying to escape you should be able to knockout enemy guard with your hands ofc it doesn't make him dead on 1 hit, it will be more like stun making enemy stay on ground for let's say 10-20, 30? seconds? enough to be able to take weapon from him and some magazines and if needed kill silently with knife

2. With weapon but without ammo - weapon stock, pistol hand grip

When you have weapon in hands but you don't have ammo, you should be able to hit with stock, while using pistol you can hit with pistol hand grip once again it will be only as a stun knockout not a 100% one hit kill.

3. no weapon/ammo - knife

Well knife is knife so it should work similar to hands "attack" but it should be able to kill enemy on 2-3 hits? every time stunning enemy (let's be realistic no one will stand like stone while you stab him in chest, leg, shoulder with knife) another idea to use knife is to attach it to your weapon since we already have slots in weapons to add attachments then instead of holstering weapon and then taking knife from action menu will effect just in pressing "F" to switch from firing mode to melee.

4. How

Well that's the question cos hands/knife/weapons stock or pistol hand grip should be as 3 separate additional "weapons" or as an additional currently handled weapon option, for example while having any assault rifle, LMG or sniper rifle you can press "F" to switch to melee with this weapon the same with pistol so this makes you currently decide, do you want to make noise and risk being spotted or you can just knockout enemy with your weapon and make him defenseless (keep in mind he still have his hands) and leave him or then kill him with knife.

And about knife this is something that will need to have extra weapon slot in your equipment or to be able to use it you only need to have it in your vest (in backpack it will be in-active) so it will again make you decide to use weapon or holster weapon grab a knife and do it more silently.

And finally Stealth kill well... this will be kinda awesome but still like in real life very very hard to do (the chance can significantly lower your failure and being spotted making silent kills at night covered in bushes near road etc.)and to activate it you should get special icon on enemy while you are in good range to him and then press action button "space bar" just the same way like you open doors or get in to the vehicle and finito,

TO THE ONES WHO WILL DOWN VOTE THIS:
Soldier isn't only a trained man in shooting he is trained in many branches like survival, melee fight (krav maga for example), map reading, communication protocols (what, when and how to say something on radio) so before you will down vote it and say that its not some stupid arcade game, THINK!

At the end I can only say that not only me but many many maaany players (there will be haters to I'm sure about it) will be thankful if you will add this cool feature cos in Arma 1 and 2 there wasn't any possibility to use melee and it was very big MINUS and pain in the !@#$%

PS: it would be also awesome if some real soldier will come here and say his toughs about this and how it works in real life, cos I'm only a military fan boy playing ASG tactic games, and... military games :P