Enough has been said about how terrible Resident Evil 6 is, from the ridiculous story and the lack of horror to the spotty controls, awkwardly laid-out zones, poor scenarios (driving sequences?), and unexplained mechanics (like the sudden use of spotlights). Instead, let's look at what worked; i.e., if you could strip this rotting zombie of a game of the useful portions and graft them onto a better game, which mutations would you take?
First off, the concept of four separate campaigns, each emphasizing a particular strength of the Resident Evil series, is a smart something-for-everyone approach. Ada features puzzles and (new to the series) stealth, Jake revolves around escape sequences and melee combat, Chris is a full on cover-shooter, and Leon is a tight-quarters next-generation survival-horror game. The fact that each of these individual sections is half-cooked is beside the point; there's a ton of content here, and by breaking the game down into easily digestible chapters, Resident Evil 6 caters to hardcore and casual players. The drop-in, drop-out co-op isn't bad either, though random matchmaking can ruin this almost as much as the poor AI that's supposedly on your side; far more salvageable is the use of parallel narratives, in which the various campaigns bleed into one another. The story isn't interesting enough to justify four different perspectives, but it could have been, and the intersecting co-op, in which two players may suddenly find themselves together with two more, is a neat feature for the boss fights. (It's a shame they're then so gimmicky and not at all reliant on actual teamwork.)
Second, the idea of allowing human players to take over the AI in the so-called Agent Hunt is a genius one. Mind you, it's not at all developed, and it's awkwardly integrated for both the humans -- who, regardless of difficulty settings, will encounter more monsters than ever (infinitely spawning, if they get stuck in one of the poorly laid-out and map-less areas) -- and for the zombies, who each have their own unexplained control schemes. But the basic idea of having devious players go back through to grief those who followed in their footsteps is a smart one, especially if the AI is able to actually process the various tactics humans use and to replicate them further on down the line. (Adaptive AI, pulling from all of RE.NET's player experiences.) We're not at that point yet, but as next-generation systems come onto the market with their advanced processing power, and designers continue to implement twists on a once-tired AI formula, we may have games that are challenging not because of reduced/increased damage modifiers but because of unexpected behaviors that keep us on our feet.
Third, limitless weapons have been a long time coming to Resident Evil, at least ever since it decided to abandon its low-ammo, actual survival-horror roots. In this newest installment, you have infinite inventory space for weapons -- the only thing that's limited is how much ammo you can carry. (Hopefully this will be phased out, too.) In the past, players have had to randomly stumble through each area, getting by with whichever weapons they happened to choose to bring with them, even though other gear might have suited the situation far better, had they but known. The point is not to trick the player with what they cannot possibly see coming, but to provide them with the tools they need in order to deal with everything that's thrown at them. By allowing players to carry every weapon, each with its corresponding strengths and weaknesses, Resident Evil 6 was able to throw a wide variety of enemy types at the player, particularly with the clever J'avo mutations. The sooner that players also have infinite room for ammunition (or better yet, universal ammo, ala Dead Space 3), the better, because that's when we're tested not on pointless conservation (using the Handgun against every foe, lest we be short on ammo that we need later) but our quick-witted responses, which is really what you want in an action game anyway.
Finally, I'd actually keep the one-hit killing monsters. If you're going the horror route, there's nothing more frightening than an invulnerable foe that can kill you if only it can catch you. But I wouldn't make their ability to kill you so cheesy, with quick-time events (QTEs) popping up out of the blue, poor dodging mechanics getting in your way, or a failure to communicate what you're supposed to be doing. Keep the controls fixed, not the fight itself -- we shouldn't inexplicably die because of something unforeseeable; we should die because we failed to heed the game's naturally occurring advice.
I've played a lot of indie games lately, and the one thing that I can praise above all else is their internal consistency and deliberate choices, things that keep them from going all-out with a AAA kitchen-sink-style approach, as with Resident Evil 6, a game that could've been great if it had only focused. Trial-and-error has no place in a top-shelf game like this, but at least we can all point out the successful designs (even things as small as the lovely aesthetics on the HUD), so that if we must be subjected to endless sequels, we at least eventually get better ones.