BLUF: What would removing the Disengage action and replacing it with Careful Movement (costs an additional 1 [or 2] ft of movement for every 1 ft moved, or half your base speed) look like?

The Problem

This came about when I was pondering how to make combat, weapons, and Feats more interesting. My biggest frustration was that positioning and movement mean so little in 5e combat (not necessarily a bad thing, coming from a predominantly Theatre of Mind GM, but there's still scope to make combat more interesting and dynamic). So the goals I settled on were:

1. Make positioning matter in a simple intuitive way
2. Make movement in melee risky but not oppressive
3. Remove false-decisions and add tactical choices

1. Make positioning matter in a simple intuitive way. It's clearly harder to fight enemies on either side of you and great to reward players for teamwork. The optional DMG Flanking rules are simple and intuitive for this as well as offering us more design space to engage with (features that allow you to exploit/negate flanking/being flanked, etc). However, as many have noted, it is incredibly easy to achieve for minimal risk and no cost. This cheapens spells/class features and makes the PCs much more powerful. The solutions I've seen tend to fall into either: make flanking weaker (+2 to attack roll, etc) or make movement more dangerous (Opportunity Attack whenever a creature within Reach of another creature moves). Keeping in line with simplicity and 5e's general aversion to static boni/penalties, let's explore the latter.

2. Make movement risky but not oppressive. OAs provoked at every movement within Reach certainly makes things more Risky (and makes getting knocked Prone a lot more dangerous, which I like), especially against monsters with Reach (which I'm a big fan of). However, we've now made it a little too oppressive and contrary to our design aims of dynamism. Apart from characters with Bonus Action Disengage, we've now made it less likely characters will move in combat once they've reached the enemy due to the false-decision of using your action for Disengage (expanded below). The fantasy of warily circling an opponent for tactical advantage is gone, except for Rogues and Monks. Now we can look at previous editions where you could substitute your entire movement to Shift or Step 5ft without provoking an OA, however, I feel that example slows you down a bit too much. So we're looking for something between a single 5ft Shift and your full 30ft Speed, that everyone can do.

3. Remove false-decisions and add tactical choices. Disengage is a false option for any character other than Rogues or Monks (except in very rare circumstances, and we're looking at standard combat). It has been semi jokingly stated that the best status condition in 5e is Dead. Spending your action on anything other than attacking is rarely worth doing which is why classes/Feats that allow Bonus Action abilities are so valued. We still want a cost to it though although, as discussed above, all your movement for 5ft is a bit much.So how do we make Disengage a trade off that doesn't cost an entire turn of damage without giving everyone it as a Bonus Action and undermining Cunning Action, etc?

You can't (easily).

The (proposed) Idea

To start: all movement in Reach (including standing from Prone) provokes an OA. Next we want a simple optional movement penalty anyone can take (Cautious Movement) that isn't as severe as a 5ft Step but still makes it not trivially easy to manoeuvre. Taking inspiration from 5e's existing movement rules we see 2 options:

1. Costs half your base movement (e.g. standing from Prone). This allows most characters 15ft to manoeuvre around a Foe or leave Reach (with a bit more for fast characters. This means a character charging into melee will be unlikely to have enough speed to manoeuvre into a flanking position that turn, but will be able to do so on their next turn. It also means standing from Prone in Reach would cost 1/2 your movement but provoke an OA or all your movement done Cautiously, which seems fair and presents an interesting tactical decision.
2. Costs an additional 1ft (or 2ft) for every 1ft you move (e.g. Difficult Terrain, swimming, climbing, etc.). +1ft for every 1ft (stacking on Difficult Terrain) seems the same as above at first glance but has some slight but interesting differences, most egregiously, leaving an enemy's Reach becomes trivially easy for an additional 5ft (unless the enemy has 10ft+ Reach) and being better for characters with higher speeds. +2ft for every 1ft makes things a bit more demanding, with it costing 15ft to move 5ft and out of Reach. It would also limit most creatures to 10ft movement within an enemy's Reach for manoeuvring. Again, characters with additional movement speeds or Bonus Action Dash are rewarded, which feels right and tends to balance out the loss of Bonus Action Disengage on these classes. 

For both options, limiting the other creatures movement/speed suddenly becomes much more important to stop them flanking or escaping Reach but unsure whether it trivialises Disengaging. On the flip side, it makes approaching larger enemies with Reach boni suitably riskier: is it worth slowly but safely approaching or getting the tank to charge in first and soak up the OA so the rest of the party can follow on? On a different note we could also wrap the house rule that Stealthy movement costs double into Cautious Movement if desired, allowing Feats and features to tie off that as well.

Feats & Feature ideas. Feats, class features, or spells that grant Disengage for free could be changed to prevent Opportunity Attacks or allow faster/cheaper Cautious Movement. Certain Feats could negate the enemies' flanking benefit, allow you to do something special if flanking/flanked, or allow allow OAs even if the enemy Cautious Moves. If we go the route of Stealth needing Cautious Movement, the Rogue could have a feature exempting them from this requirement and do a similar thing for the Skulker feat.

Thoughts? I'm going to play test with my group (1/2 movement option) and see how it pans out but wondering if anyone had tried anything similar or could see any immediate flaws. Is it worth it to get flanking and more danger in melee? Has anyone tried anything else that worked?

It's kinda sad that you never got any replies for this, because it's a great idea and I'd be excited to hear how it turned out in actual play! 

So we remove Disengage as a basic action and add "moving from one threatened square to another" to the list of OA triggers, with the stipulation that spending double movement negates this. For Rogues, we could actually leave things the same, letting them keep the old Disengage as a special rogue bonus action.

In Player Options - Combat & Tactics from 2e, there is a mechanic called "retreat" where if you hit an enemy in melee and don't get hit back, you can forcibly move them 5 ft backwards. In 2024 D&D, this is kinda covered by the push maneuver, but in this context, maybe it could be a special type of attack action, which triggers an OA, but moves the target if you hit and they don't.

Removing Disengage seems like an odd idea, since only Monks and Rogues can disengage as a bonus action, and those classes also have other uses for their bonus action, so there's an opportunity cost as well. For anyone else, they need to spend their entire full action to disengage, which means giving up on almost all options for attacking and spellcasting. Are you really getting so many PC's disengaging that it's frustrating you for some reason?

Also, positioning means a LOT in 5e because a lot of spells and other features either effect an area which could include friendlies, or have a limit to their range. The thing which is causing that to be irrelevant is the use of theater of the mind, where positioning becomes arbitrary and confusing. I played in a short-lived campaign which used TotM and even in the few combats we had, I was often confused by enemies attacking me in melee, and not moving away from me, but apparently also not being in melee so I didn't have disadvantage on casting ranged attack spells.

I do believe these rules were designed around martial classes having low tactical decision making rather than spellcasting and ranged being the main force to move. And while they do make movement important I think that martial classes having their own way to make engaging harder and disengaging easier would help with the monotony of playing them and help curb the oppression of some high level spellcaster builds.

I got an idea for how to further develop this homebrew mechanic, which is by overhauling Attacks of Opportunity, though I will say right away that this departs from standard rules significantly.

1. Make OAs not take a reaction.

2. Limit OAs to once per creature's turn (so kinda like a legendary action)

3. On hit, an OA reduces the target's movement to 0.

Together with the Disengage changes, this makes martials much better at tanking. Creatures wanting to attack the martial's allies, have to pick between spending double movement, or risking being stopped altogether by an OA. Removing the reaction cost, but limiting it to once per turn, makes the martial able to block a multitude of creatures, rather than threatening one and then the rest run through.

This also means that OAs no longer compete for reactions, so that fighting styles like Protection don't clash with it. Obviously, this kind of mechanical change would require some fine tuning and changing of other mechanics. Since Disengaging is now so much easier, maybe we could bring back the close range disadvantage for reach weapons from old editions.

Just a few ideas.