To try and promote more movement in my game I'm considering a ruling where all Opportunity Attacks have disadvantage. Are there any ways this would significantly change the game? Party consists of two bards, a blood hunter, a drakewarden ranger, a fighter and a druid. Thank you!
The wizard won't like it.
The big negative impact this would have, in my view, is that it makes it significantly harder to maintain a defensive front line. If enemies can pretty much run past the fighter/paladin/barbarian with impunity to reach the low-AC low-HP casters in the back line (at least, without expending their action on a Disengage), then the casters are going to be taking a lot more knives to the kidneys, and they typically don't like that.
The traditionally high-AC classes like fighters and paladins want to encourage enemies to attack them in favor of the weaker targets, and the biggest tool in their toolbox is punishing anyone who tries to ignore them. Making it harder for them to dole out that punishment is going to make the casters feel bad as they spend more time laying on the floor, and make the front-liners feel bad as they're failing in their party role.
This will favour mobility based builds
Your group's specific builds determine how much impact this will have.
For example, a bard can be College of Lore focusing on battlefield control and spellcasting, or College of Valor bard getting in there. A fighter can focus on ranged combat with Sharpshooter and not care, or on melee and movement. If the fighter is a Polearm Master, it will weaken them, if they are Mobile and built around high movement rates, it will strengthen them significantly.
The change looks symmetric on the surface, but it can be quite assymetric if your group is heavily optimized for this or relying on making opportunity attacks. If they are not, I do not think it is "game breaking".
I have not played with this changed rule, and like most things that change a fundamental aspect of the game, the best method may be to try it out, with the up-front proviso that you'll take it off the table again in case it does not work well. That way, your players will be warned to invest in building characters around it.
Not a direct answer about balance, since that's been covered, but if the real problem is that tank-intensive combat is getting boring, you might get more fun movement by changing the structure of the encounters rather than changing the mechanics:
- Perhaps a running battle where PCs are trying to get some NPC to safety?
- Or maybe a complex kill-zone trap room where PCs try to simultaneously pull several widely-separated levers? If one or two of the levers are obviously within the enemies' area of attack, then the tanks have to abandon the wizards.
- Maybe the dungeon is collapsing, and PCs have to fight their way to the exit?
- Or a magic weapon / shield that provides an increased benefit when the wielder is moving?
- Or provide movement-enhancing spell scrolls / magic items?
In my experience, once players get in a few sticky situations where there are obvious benefits to changing tactics from their norm, they start getting creative about their tactics, and start looking for opportunities to re-use those new tactics.