It would seem to me that these are things that any class can declare to do. It doesn't sound like an actual class feature or ability.
Can't a wizard simply say: "I gag the captured bandit"? So how is that a class feature?
So far I'm not seeing anything really unique about this subclass. You mention that the Thug could be both a criminal and a none-criminal character (bodyguards, bounty hunters, secret police, and spies), but I don't see anything in the abilities that fit a good character at all. And is a bodyguard really a thug? Is a spy a thug, or a bounty hunter? I don't see it.
More importantly, I don't see how a character specialized in abduction would even function in an adventuring party.
Thanks for taking the time to look this over. You have some interesting viewpoints, which I'll try to address.
Yes, a wizard could gag a bandit that is already captured, but only a Thug can do this as part of a grapple action, to better capture that bandit in the first place without him yelling for help. This is Exploration Pillar stuff, to build on the Rogue's traditional roll as a scout. Sneak ahead, nab a sentry, and bring him back for interrogation (aided by your intimidation buff).
As for "good characters" I see the Thug as occupying more or less the same moral ground as the Assassin, so there's official precedent. I mean, a bounty hunter's whole profession is abducting people... "Wanted: Dead or
Alive"? Secret police make people disappear, spies (think less James Bond, more Jack Bauer) exfiltrate hostile targets for extraordinary rendition, etc.
I think the grapple buff feature is pretty applicable to the adventuring life. There's the aforementioned sentry nabbing, battlefield control (grab a dude and throw him into your spellcaster's AoE spell), enemy spellcaster lockdown (there are only 9 spells in the PHB without a verbal component), and so on.
I think 'blindfold' and 'gag' create the impression the Thug is actually tying something to the victim, which I don't think is what you're getting at?
Yeah, labeling the effects with noun words like "gag" and "blindfold" is probably confusing, so I should change that.