I am not particularly hyped by this sort of feats, but I think they would be fine, I wouldn't have much problems allowing them.
Basically, these two feats in particular allow a character to "upgrade" their skill to replicate the effects of a spell.
- IMHO, charmed and frightened supernatural conditions and should not be the result of a skill check. An NPC may find you charming, or be frightened of you. I may even borrow from those conditions - but I want more flexibility to deal with results than the conditions allow.
I don't think that
conditions themselves are supernatural, in general. And these two feats seem to be intended as non-magical in nature.
It's true that these feats aren't flexible. If a PC tries to charm or intimidate an opponent with normal means, typically the DM is quite free to handle the situation the way she wants, while these feats grant a clear-cut result.
- Many, if not most, players will expect to follow the wording of the text which is very mechanical. "If you do X you get a contested skill check. If you succeed Y condition is imposed on the target".
I think at least the DM can choose to give (dis)advantage to either roll, but once the opposed checks are done, the effects should pretty much conform to the description of the condition.
- A PC is far more likely to win an opposed skill check than a target failing a save. Based on a quick check of stats from a spreadsheet I downloaded, creatures in the MM have a +1 to their insight check. At a certain point, depending on the build, a PC is guaranteed to win every skill check against the vast majority of creatures.
This is probably true, especially if the PC was already proficient and now also got double bonus from the feat.
Rather than just drop the whole idea, if the problem is that overall they are too
strong feats compared to others, I'd just remove the ability increase from them, to tone the feats down. The various restrictions could also be revised, e.g. limiting Diplomat to humanoids only.
- In addition, certain monsters have counters to spells. Either the capability to literally cast Counterspell, magic resistance, legendary saves. No such mechanic applies to contested skill checks.
Yes, and this makes a non-magic ability that replicates a spell definitely better than the original spell.
I think this is actually a good idea, much more interesting that granting the PC the ability of casting Charm Person or Cause Fear. Because in general the game already has quite a lot of magical abilities... IMHO it's better to add non-magical options rather than even more magical ones! In addition, it can serve low-magic settings or "grim'n'gritty" adventures better.
As for the action economy of Menacing, I am not really sure... I think it's ok as an attack, but if you change it, what are you going to replace it with? A bonus action? The problem is that as a bonus action it then gets in the way of other class abilities.