Looking for a New Co-op/Multiplayer Game to Play. Need Suggestions!

HALO 5

xbox one only, 29 days to go before I get it and finish it in that day...

Anyway, If he has a lot of time to waste, play warframe, 1 day to build a weapon, 3 days for warframe, its a event to win, not pay to win.

Get Just Cause 2, It has a good campaign, I still play it, almost near 90% done.. And when your done with that, you got the multilayer mod, its full of random things.. Ramming, races, GTA 5 but its mixed with goat simulator..

That is all I can say.. the rest are games that are in-depth campaign.. e.g. halo, portal,fear 2 (the best/most jump scare one there is, fear 3 is just good mechs, but dull) Mirror's Edge, NOT A HERO is a very good one, its not multiplayer, but its fun, Its If you like puzzles mixed with fast action gun shooting rabbit lord games..
Orcs Must Die is fun to play, With friends mostly, Project zomboid is good if you can set up a server, Or find a good one with low amount of people.
Serious Sam 3: BFE Is a fun game, Action with a lot of random stuff, And if you get into it, there around 100 Easter eggs to find inside of it.
Left 4 Dead 2 is very fun to play with, on steam the mods are so funny, from racist to Very random cat zombies mod.. Or rainbow blood ect.
The binding of Issac is good, but as long as you don't mind going over and over doing the same thing again, then you will like it,

And thats all I can say..

 

If you both own a 3DS, I can't recommend Monster Hunter 4 Ultimate more.
If not, Hammerwatch is pretty good, and so is Risk of Rain. Vindictus is good if you're looking for a skill-based MMO. Although stats play a pretty large part in vindictus, so does skill, I've seen max level players go down against bosses that can barely touch me (and I'm currently 17 levels and a few armor sets away from the level cap).

 

I own/enjoy most of these games so I'll just provide an alternate view here.

Brawlhalla - How many people are on fluctuates a bit, also don't expect a smash bros clone, it's not.
Magicite - Have friends to play it with or don't bother.
Nidhogg - Low seller due to the initial cost, not sure it went down, but given it's original '2 player only' multiplayer issues, it wasn't as huge as it could have been. I haven't purchased it yet, but have kept an eye on it, it does look better recently though.
Unepic - Fun, need allies to play with, the singleplayer is rather boring compared to the multiplayer but the multiplayer requires going back and replaying levels with your friends, a lot of the levels are very tough, like most DnD(stat RPG)-inspired games, it's not very forgiving on bad choices, make your choice of character early and stick to it.
Hammerwatch - Has a few annnnnnnnnnnnnnnnnnnnoying bugs that will make it so that you don't get too far unless you get around them, most of them were fixed 3~ months ago, but I believe a few crash bugs are still in.
Minecraft - ( ͡°╭͜ʖ╮͡° ) Seriously though, if you do it play it with mods. The unfortunate thing about minecraft is if you didn't get it early you're paying an arm and a leg, unless you... acquire it by alternative means. You also want need friends to play with unless you're some fucking super aspie.

I'll throw some more in here.
Dota 2/Smite/LoL/HotS - Honestly picking up a MOBA to keep you busy is a good idea if you haven't tried one, there are MANY around and in many cases while depending on which you pick you need to be able to tolerate that community (PM me if you have questions as I've played em all), it usually will sate your gamer's blood for more complicated gameplay as you go, the only thing keeping it back is the fact that you have to play with 4 other dickwads who are probably just as shit as you if not worse. (I see you already play LoL but it has to be thrown out there) [There's also Magicka Wizard Wars that 'loosely' is considered a MOBA]
If you want something to just fuck around for a bit Demigod (Released way before LoL/etc, but after DotA (obviously)) is a fun little moba to mess around in, but not very much so for a larger scene than messing around against bots or with 8~ friends.

[ARTS] Marvel Heroes 2015/Path of Exile - The former if you have a casual friend to play, the latter if you have 'gaming' friends to play with as you've stated, especially for uncovering fun mechanics. If you enjoy either of these I can also suggest Grimdawn , I've heard nothing but good things from PoE enthusiasts (I myself am one), I can ask AJ as he owns it if anything changed recently, but as of 9~ months ago with a good overhaul I've only heard good things and the few streams I watched showed it coming along nicely. Also while I am NOT a big fan of it myself, Diablo 3 is enjoyed by quite a few people especially after one of the big updates, this will however require a decent monetary investment.

Dungeon of the Endless - Very fun, unfortunately has terrible problems with connection dropping multiplayer and players cannot rejoin when it happens. They fixed ONE of the issues, but there are many and they really kill the mood.

Don't Starve Together - Very fun, no overall problems with it, can use dedicated servers if you're into that. Would suggest 4+ people though if possible, it gets more 'fun' with the chaos of quite a few people, whereas it's sort of more manageable when there's only 2.

Risk of Rain - Pretty fun, no overall big issues I can think of with it, would suggest playing with both of you, very fun for 2 player.

Robocraft - Recent update allows just using other people's inventions without having to 'make' them yourself, this could be good, this could be bad, haven't played it since this update. Pretty fun make cars/etc and shoot down your enemies. CTF very fun.

Terraria/Starbound - Pretty fun for 2-10 people (both of them), Starbound is funner with mods the second time (especially side quest/dungeon mods). (Expect a week max out of these)

Age of Empires II - Do you even, bro?

Rocket League - I haven't played it on my own account but watched my little brother play it and played it on his a little bit, it's quite fun. Cars+Soccer = Who knew it would be fun.

Torchlight II - Another ARTS, pretty fun, second runthrough with mods is also not a bad idea. My only gripe here is it suffers heavily from 'Oh you went on without me?' syndrome, so make sure if it's just you two that you only play when you're both there, if you add more people in this becomes more difficult, so it's good for 2-3 people at most.