When I first saw monster stat blocks in the D&D Next playtest, and again upon the release of the Monster Manual, I was surprised at how much of a return to 2e-and-earlier form they resembled. They have some of the formal action presentation of 4e's creature blocks, but they're much simplified, and they're nowhere near the eye-bleeding-dense lists of skill bonuses and feats that 3.x creature blocks become at even low-mid levels. Now, this has some serious plus sides - ease of use is way, way up.
On the other hand, I miss the way that the combination of stats, actions, and traits in 4e sculpted encounters and built an engaging tactical puzzle. It was definitely possible for this to go too far, and running 4e monsters at peak tactical efficiency pushes the mental advantage of 3-6 players up against one DM to its breaking point. I'm not the tactical genius of some of the DMs I know, able to crunch all of my options and pick the most devastating one for any situation. The game still works well, but the sharper the DM is in 4e, the more the puzzle crystallizes, and I choose that word because it's as beautiful as a mathematical proof covered in the viscera of your enemies. (Jeopardy question: What is best in life, Conan the Logician?)
What I'm getting at, then, is how to slow-roll some of what worked back into 5e, without exploding into excess complexity, and generally holding to the design aesthetic. PCs still have a lot of the tools their 4e counterparts do, exemplified in the Battlemaster archetype and the Sentinel and Shield Master feats. This also highlights, of course, just how big of a gulf there is between a campaign with feats allowed and one without, or a D&D Basic and straight PH D&D campaign. They all work, but they work very differently.
As you may have gathered from the title of this article, I'm opening what may be an ongoing series with defenders - what 4e called the Soldier monster role. For a non-mechanical approach to what I'm doing here, go read +Ben Scerri's advice on the matter. Okay, really, just treat his article as Step One; if you try that and it doesn't go far enough, I'm here for you as Step Two. (If you're currently formulating a nastygram about how anyone who needs mechanics to fulfill combat roles is a bad DM, then this isn't for you, and also I don't know how you can even read this post from way up on your high horse.)
There are a lot of different ways to make a creature just a little more of a defender, so I'm recommending that you pick just one - two at the most - of the following features to add to the creature(s) you want to set up as the defenders for that encounter group. In general, don't set up all of the creatures in a group as defenders, unless you're really, really sure you know what you're doing - the point is to have varied tactics by creatures in a group. You'll also notice that I'm not pushing numerical adjustment; obviously, you can increase an individual creature's defenses to your heart's content. That doesn't change its behavior, though, as much as it gives you more complicated numbers to track - like remembering that this hobgoblin is the one with 2 more points of AC, but otherwise uses the same stat block.
Also, these are not all thematically appropriate to all creatures! Not even close. You'll need to carefully consider which trait or traits fit your mental image of how the creature fights.
When the creature hits with its attack, the target must pass a saving throw (usually Strength, but maybe Dex) or fall prone.
This makes the creature a defender by making it harder to move away from the creature. It is very light area control, taken from the Dire Wolf.
Alternately, make this a bonus action whenever the creature uses the Attack action, and it's one-third of the Shield Master feat. It may be appropriate to allow this for ramming creatures that do not carry shields. I don't know your triceratops-riding life.
When the creature hits with an opportunity attack, its target's speed becomes 0 until the beginning of the creature's next turn.
One element of the Sentinel feat. Giving creatures just one piece of a multi-part feat seems like pretty good times to me - what's sauce for the goose is sauce for the gander.
When an enemy within 5 feet of the creature makes an attack against one of the creature's allies, it can spend its reaction to make a melee attack against that enemy.
Again, one-third of the Sentinel feat. Revenge tanking is best tanking.
The creature has advantage on saving throws against spells cast by enemies within 5 feet of it.
One-third of the Mage Slayer feat. This only makes the creature a better defender in that it encourages it to get in the faces of PC spellcasters - ultimately this trait is better for harrying back ranks than engaging the PCs' melee brutes, unless those melee brutes get the job done mostly with magic.
When the creature hits with an attack, it can attempt to grapple its target as a bonus action.
This is like the +prone trait above, but with much more convincing stickiness. The soldier is fine with using the prone condition to set up an ally's attack, but that's not its purpose in the fight.
When you have a creature grappled, attacks against you by any creature you do not have grappled suffer disadvantage. When such an attack misses you, you can use your reaction to force the attacker to repeat that attack against your grappled target.
How to make tentacled horrors or giants even worse to fight, basically.
When an enemy starts its turn adjacent to the creature, it must pass a Strength (or whatever) saving throw or be restrained until the beginning of its next turn.
Or push this onto a bonus action by the defender, so that it's only restraining one creature at a time. Obviously, this is also about very serious stickiness.
The creature gains a new attack mode that, on a hit, either knocks the target back 10 feet or pulls the target 10 feet closer. Attack bonus and damage should be scaled to whatever makes sense.
This is about just controlling the geography of the battlefield, either pushing targets so they are farther from the creature's ranged-weapon allies (or just separated from their own allies), or pulling them toward the defender so that they have to suck up an opportunity attack to go do their thing. Repelling blast and lightning lure are both good examples here. Overall, I've been surprised that 5e gives a fair amount of forced movement to PCs and incredibly little to monsters.
As a reaction, the creature imposes disadvantage on one attack roll an enemy makes against an ally adjacent to the creature.
The grievous flaws of the Protection fighting style matter a lot less in NPC hands.
As a reaction, the creature can prevent X damage to an adjacent ally (possibly redirecting 50% of it to itself).
This is particularly heinous if the creature also has regeneration, and strongly pushes the PCs to kill the defender first. Assuming the defender has the hit points, AC, etc., to back it up, this is one of the most straightforward expressions of the whole idea.
The creature has resistance against all sources of damage that are not Sneak Attack or critical hit damage.
This should push PCs to gang up on the defender (to get Sneak Attack rolling), which is effective as long as its positioning and/or damage output are such that they can't bypass it. There may be better phrasings of this idea, but I'm shooting from the hip a bit here.
The creature gains a javelin-on-a-chain or similar tethering attack. When it hits an enemy with its tethering attack, the enemy must extract the barbed javelin (or whatever, as long as it requires effort or inflicts more damage) to move more than 10 feet from the creature.
Yeah, okay, it's totally Scorpion from Mortal Kombat. It's also a pretty great image. Depending on the in-game justification of this feature, I'd potentially let a PC attempt a Strength (Athletics) check to pull the creature off-balance or drag it closer.
That's twelve options - probably a good enough start for now.
What About CR?
I'm not applying CR modifiers to these traits, because I don't award XP based on CR of monsters defeated (and you shouldn't either, it's the least interesting rubric), but as a rule of thumb, I would suggest that for every two creatures you tag as a defender, treat one of them as 1 CR higher. It's inconvenient for the PCs, but there aren't big shifts in damage output or individual defenses going on here. If you super care, there's a table to sample traits in the DMG to use as a guide, but that's left as an exercise for the reader.
In implementing these at the table, make sure to include descriptive elements - such as the postures that the creatures assume - to suggest that there's something unusual about the defender(s), especially the first few times you use these. If your PCs are already familiar with 5e, two seemingly-identical creatures will be presumed to be the same if they don't take steps to distinguish themselves, and the point is to add wrinkles to PC decision-making, and thus behavior. If you're adding the foundations of tactical puzzles to the game, then make sure you add tactical clues as well.
Depending on how these ideas stand up to testing - and please, if you use these, let me know how they go for you! - I'll look into adding traits for other monster roles, though I feel like brutes, skirmishers, artillery, and controllers are fairly broadly represented as-is, and lurkers are in an okay place too. It can't hurt to have a collection of traits to work from when you're building new creatures and want them to work a certain way in combat.