Final Fantasy 15
Final Fantasy 15 is a wonderful, emotionally-memorable roadtrip. It feels like you and a bunch of buddies going on an adventure that just happens to involve saving the world, killing loads of beasts, and maybe doing a spot of cooking. At the end of the game, though, you get to transform that car of yours into a freakin' jet, opening up the game even further. As is FF tradition, there are some secrets to find post-story too, so it's well worth sticking with.
Until Dawn (2015)
When various members of the GR+ team first finished Until Dawn we just assumed it was a numbers game. How many people survived your playthrough? Oh, like four. It really isn't. The choices you make in Until Dawn aren't merely binary decisions that kill off its unfortunate teens at a pre-set point - they genuinely change the whole story. You can miss massive chunks of Until Dawn by making a seemingly innocuous choice near the start, or witness some amazing story-telling by chancing upon a single diary note. As such, replaying this one - having seen the ending from one perspective - is absolutely essential.
The Witcher 3: Wild Hunt (2015)
To a lesser extent than Skyrim, The Witcher 3: Wild Hunt is more about exploring the actual world than progressing the story. Sure, the main plot is ace, but the real gems here are the superb side-quests that distract Geralt for hours on end. So, finishing the main plot simply frees up the player to explore some of the juicier stuff. Plus, the amazing expansion - Blood and Wine - follows on directly from the central story, so you need to (well, not technically) complete that before experiencing a whole new chunk of Witcher. As if this game wasn't long enough. And as if you're not still playing Gwent instead of polishing off the plot. We see you!
Assassin's Creed 4: Black Flag (2013)
In many ways, finishing the main story in AC4 sets you free to do all that delicious piracy that makes Black Flag truly enjoyable. While much of the post-game will involve hoovering up various Animus shards and sea shanties, there's actually a whole other story going on with side-quests like Templar Hunts. You probably shouldn't attempt to take on the Legendary ships until the Jackdaw is fully upgraded either, so you can take your time, post-credits, buffing your warship (ahem, so to speak). Got all the post-it notes in Abstergo? Better collect them too, and have fun looking for Easter Eggs. Sure, Syndicate and Unity are newer games, but there's more delight in Black Flag's post-story, so it stays in this list.
Hotline Miami (2012)
Masks are the key to this one. During your first playthrough of Hotline Miami you'll unlock a bunch of animal masks, which have drastically different effects depending on which you wear. Going back through the game (and it has a pretty circular plot anyway) allows you to try levels using different head wear; forcing/encouraging you to switch your tactics. Given the relative simplicity of the game, it's amazing what a startling effect a rule change can have on the way you play. Some masks help - one prevents dogs from attacking, for example - while others hinder you by dimming the lights. Rasmus the Owl even allows you to spot secrets that you may never have been aware of during your first run-through.
Batman Arkham City (2011)
In some ways, Batman: Arkham City becomes an even better Batman simulator once you've completed the main story. It certainly takes on a whole new tone and provides one Hell of a poignant, playable coda to the campaign. Following that ending, the city goes on, but is irrevocably changed. The fallout among the criminal fraternity is palpable in their dialogue, expressions of mourning, confusion and disaffection overheard as you glide by on patrol.
And although things are quieter in the stillness of that post-endgame snow, without the distraction of the next story mission to push you forward, the sense of simply 'being' Batman, doing what he does, night after night, is amplified immensely. You'll stalk the rooftops. You'll scan the streets for signs of trouble. You'll follow up loose ends and chase investigative leads at your own pace. With the main story's cast of characters greatly diminished, it's a lonely, reflective experience. But that's what being Batman is all about.