My choice right now would be: If you move past someone with a readied melee weapon, the area within reach of the opponent is difficult terrain. Alternately, you may move at full speed by being disadvantaged until your next turn.
Disadvantage provides a much cleaner mechanic for the set of penalities that I had proposed earlier this year, for a similar mechanic, especially in the way it does not stack and has other repercussions. Moreover, unless you are already disadvantaged (admittedly an edge case that would seem odd when it does come up), it has bite. That means if you run past the fighter to whack the wizard, you have less chance of connecting. (Presumably, it would also apply to a saving throw if the wizard responds with a spell, too. Don't have the rules handy.) Finally, it is something that applies right now, to the creature taking the action. It's thus easier to remember without interrupting the flow.
Then for double movement through threat, to avoid it, would be most creatures' first choice, but hard to achieve in many cases. A creature will almost always need to spend one round working around the fighter, taking a shot at him, then peel off to go after the rear rank. That one extra turn makes all the difference. Given room to do a fighting withdrawal, the fighter can slowly retreat before the pack of goblins, keeping them slowed down. That's something interesting for the fighter to do with his movement, especially if he wants to finish off one foe before moving back a bit. Then for creatures with lots of movement, it's an abstract way of handling "tumbling" through the melee.
I think this is actually almost as easy in TotM as it is on the grid. If the fighter has room and says he is going to keep the foes bottled up in the corridor, then any foe trying to get to the soft squishy parts several feet behind him can try to maneuver the fighter into a corner or take their chances with disadvantage. If a kobold monk with super speed is on the scene, he can burn through and kick the wizard despite the fighter.
How much move does it take? On or off the grid, most of the time that fighter is going to control at least three squares that must be taken. At a corner or such, it might be only two. Make a judgement as to which, creature can take the movement penalty--or not and suffer.