That being said, the combination of Misty Step + specific offensive spells is definitely very powerful, if not OP. Use all movement to move in the middle of enemy group, cast Arms of Hadar or similar spell, misty step away (used and abused in BG3). Whether it is OP or not really depends, are there any actions that are not spells, but have a similar effect? Andi f yes, what is their "price", do they expend an item/slot/charge or something?
I do not consider that problematic, when you are spending the requisite limited resources, in a system where your spell slots actually mean something of value (not BG3). If a caster wants to burn their slots that quickly on at high-burn high-effectiveness, well, they're going to burn out faster than others who don't, and the price will be well paid overall. It's not problematic or unbalanced. In terms of what other abilities exist that aren't spells... well, there are other class abilities, and their paid cost is generally the same as spells; they are a rest-limited resource.
You know what is? Fighter-paladins who can, in the right conditions, blast off eight (
Eight) spell slots in a single turn, all converting into guaranteed, rarely resisted damage, and all perfectly legally. I make that example, because the main "official" response for why the bonus action casting limitation exists, from WotC, was to make a statement about it being a gate against novaing spell slots... Said while even pure paladins can nova three spell slots per turn at bare minimum, freely.... or, to use a more pointed example, walk up to a big bad monster, pump two spells slots into it, and then misty step away out of its range, perfectly legally. If the wizard wants to spend two of their limited resource slots in one turn just to balance their standard offence with some precautions, that is in no way over-balanced. For your example, though... I don't see it as being any grave difference in benefit over walking part way into the group of enemies, using AoH, and then walking the rest of your movement back out again, because, you know, AoH is reaction denial. Don't just consider what you can do with this; consider it in relation to what you could do without it as well and measure them. The difference is, in nearly every case, minimal to minor. Healing, as mentioned below, is a consideration... but Ill answer that below.
Healing Word into a full spell is extremely potent in terms of action economy and Healing Word into Cure Wounds significantly increases the pace of healing at lower levels.
In the lower levels, yes... where your spells lots are very tightly limited, and burning two in a single turn is a big hit to your total long rest resource pool, when you might be having to get through two or three encounters in a day. More than acceptable for the cost. In the higher levels, where HW truly shines as the absolutely worst scaling spell in the game, then it's positively a detriment to be using spell slots on it except in emergencies... it doesn't even pass muster as maintenance healing,, past level 10, compared to the by turn damage output that parties will generally be facing. So if players want to, they can certainly weigh that cost themselves, without needing an extra, counter-intuitive rule that stands outside the general design philosophy of the rest of 5e.
I wouldn't argue that it's broken but being able to do so is a substantial buff.
Rather, I would say that HAVING the rule in place where it doesn't belong and has no business being is a substantial and utterly unwarranted handicap on casters who do not need it. The game's balance is not served by the existence of that rule.
Bonus action spells for caster classes are generally balanced around the idea that they don't have a zero time cost. If you do set them to have a zero time cost then you need to nerf pretty much all of them because that's a fairly major buff. Any that you don't have to nerf are either edge cases where it doesn't really matter that you can't cast a spell that turn or spells that probably need a buff.
You're going to have to explain to me where you're getting this talk about 'time cost'. They have a time cost; they take a bonus action, which is a specific part of your turn, and of which you get one and only one per turn. That is their time cost. There is no need for the implementation of further rules on top of that, when the existing structure already fully defines and limits their use.
In terms of the specifics you've stated I think there is definitely a case to be made that both reactions in your own turn shouldn't be prevented nor should action surges.
Or, rather than creating even more niggly specific rule minutia to define exceptions hither and thither, they could just abandon that one rule that sits there over the top of existing definitions, and sticks out like a sore thumb as a rule that clearly doesn't fit in with the entire rest of the 5e design style, and was only put in as a legacy response from older editions, due to preconceived (and historically founded, I admit) notions of casters as badly overpowered especially in their ability to nova powerful spells in rapid succession, which is something that has already been taken care of and is not an issue in 5e at all and does not need additional rules shackled onto it any more.
Right now, I'm involved in 5 different games, with various groups of friends, and not a single one of those games has not unilaterally decided to discard the silly bonus action spell restriction. Most of them have been running for several years now, and believe me when I say that it is not the spellcasters who are the ones shining the most, in any of those games. It's pretty well balanced, for the most part... the most class-centric comments tend to be about the barbarian that cannot be killed, at one table or the monks at another; there are multiple sorcs, wizards, warlocks and clerics scattered about the groups, but they aren't the ones stealing the show of gaining excessive extra value from anything. The cases of actual bonus action spell and action spell use are rare-ish. Not entirely uncommon, but not consistent either. But they do have the comfort of the freedom not having to stress about whether they can or not, because every spell just takes up the portion of your turn economy that it says it does, and that's all you have to worry about. The level brackets for the ranges currently between 5 and 13; not quite in the end tiers yet anywhere, but getting there.
I'd seriously just invite anyone to try it; play a campaign without that silly rule. I swear; you will not miss it, and you will not go back... because you will see that there is truly no need to.
