Being very new to 5e I've been reading through a fair amount of material trying to understand the key changes. One thing that stood out to me early was that it looked like builds relying on multiple AOOs with rider effects don't work in 5e because AOOs take up your reaction and I didn't see an equivalent to combat expertise. Then I noticed the Tunnel Fighter from Unearthed Arcana. This solves the problem of multiple AOOs (though on a general note it seems like this whole concept is harder in 5e because the trigger for those AOOs is more limited, so it will come down to positioning I think).
The next problem was finding a reliable "lock down" mechanic. What I've come across so far are the invocations, again from UA. Three different ones, two of which set speed to zero and the other knocks prone. Now we are talking.
The last piece of the puzzle is expanding reach so as to "threaten" more area. Again, one of the invocations from UA does this, which might make it the better option. Beyond that I haven't found a ton of options, though the mystic's giant growth looks nice (+5 to reach and +5 more for two power points). Putting it together:
1 Fighter/2 Warlock/1 Mystic
Each turn you have to burn your bonus action for Tunnel fighting, but with giant growth going this should give you 20' reach on claw of acamar with unlimited AOOs, each of which reduces the target's speed to 0 until the end of your next turn if you hit.
Thoughts?
Edit: Doing some more digging on this point I realize for a very simple version of this it's even easier to pull off. Level 1 Human Variant Fighter can take Tunnel Fighting style and get the Sentinel feat. It isn't perfect, but if the goal is to stand in front of the team and not let the bruiser bad guys through, it works (and even the reach is handled by just using a reach weapon). Getting Polearm Master at 4 could also be an option with such a build as it would allow for stopping baddies further away (doesn't matter if the goal is just to keep them from reaching your back line and they intended to go through your reach area anyway, but does matter if they decide to swarm you and you use this to mitigate that by stopping baddies without reach in a place where they can't hit you).
Still, I think a good argument can be made for the basic build I listed above, especially when you take into account the Flail Mastery feat (another UA gift). Opportunity Attacks with a flail force a STR save or they get knocked prone. So with Claw of Acamar, anyone hit with an Opportunity Attack gets move speed reduced to 0 without a save, then also has to save or be prone (which, with a 0 movement speed for a round, means they can't get up).