That is certainly defensible. It lacks the feel of actual TWFing because you can't split your attacks, but maybe I should just give up on that.
Yeah, that's the trade-off for reduced complexity. If there's one major downside, it's that it is a little too similar to just swinging a greatsword.
I think that being able to split attack is the primary defining feature of 2WF. Without it, it becomes almost a cosmetic difference, except in corner cases (e.g. two magic weapons with different effects). But that unique feature is lost as soon as you would get Extra Attacks, at which point you can have at least 2 different targets per round no matter the choice of weapon.
- Rogues don't benefit disproportionately. They actually miss out on getting two chances to sneak attack, but they benefit from keeping their bonus action to Dash or Disengage.
Uhm... isn't sneak attack limited to once per turn?
I think you need to keep the bonus action to make hunter's mark and such spells not only be good with two-weapon fighting. Otherwise getting an extra attack you can always add hunter's mark to is really strong.
Anyway, I don't see much problems with Hex or Hunter's Mark. The bonus action is required only at the beginning to cast the spell (and it could have been a whole action, so I'd rather see the bonus action as something to be happy about rather than a limitation) and when changing to a different target. Most of the times this happens when the previous target becomes innocuous (dead, incapacitated, runaway etc.). It becomes action-costly when fighting lots of small enemies, but that just means these spells work better against few large enemies. I still think these spells are
very strong.
I'd be more worried about Cunning Action... not all Rogues use this on a regular basis, but certainly some of them use it all the time, and it actually feels somewhat appropriate thematically for a Rogue to combine high mobility with 2WF because both of them represent being
fast. So this is the only case when I'd be tempted to flat-out rule that you can do both things in the same turn i.e. use Cunning Action
and get an attack with the off-hand weapon.
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With a bit more courage, it would be possible also to just house rule that the off-hand weapon attack of 2WF doesn't require a bonus action at all, and it's instead part of the same attack actions. That would be a very simple house rules.
How could this break the game in favor of 2WFers? Let me see what kind of things the 2WFer could do with that bonus action, if she doesn't need to use it for the off-hand attack [this is not a complete list]:
- enter/end a Rage (Barbarian)
- get an extra attack (Berserker, War Cleric)
- use Bardic Inspiration (Bard)
- use Wildshape or heal during it (Moon Druid)
- use Second Wind (Fighter)
- activate some maneuvers (Battlemaster)
- get one or more extra attacks (Monk)
- Disengage or Dash (Rogue, Monk, some Totem Warriors)
- Hide (Rogue, Ranger)
- Dodge (Monk)
- make some ability checks or use an object (Thief)
- converting spell slots (Sorcerer)
- control a pet (Beastmaster)
- cast or control some spells (many classes)
Most of these aren't going to be done much more often than once per combat
or they actually have another cost (slot, ki or another limited resource) so I wouldn't worry about them being overpowered if used together with 2WF.
I would probably however rule against those which grant yet another extra attack.