With the permission of @Kelega from the Miskatonic Malcontent, I want to share this external link that breaks down in detail and with graphs on which traits to choose for every campaign up until TSK.

The author shows:

• What traits appear the most in each campaign
• What traits do enemies have in each scenario of each campaign
  • What is the best combination of 2 or 3 traits to cover most enemies in each scenario of each campaign
• What traits do treacheries have in each scenario of each campaign
  • What is the best combination of 2 or 3 traits to cover most treacheries in each scenario of each campaign

Warning This is full of spoilers, of course, but as Grizzled is mechanically made for guessing what the most common trait used is, if you have already been to this campaign and want to tech your deck, then this is all you're looking for.

For 6xp (Always Prepared and an additional Trait), you can either build yourself a repeatable enemy management tech or a 2-card constant +3 against treacheries.

Auto include in any Amina deck. One of the most common issues with playing a doom-based deck is that your fellow investigators will complain that your cards are adding to the doom total. Deafening Silence solves this problem by moving the doom off of your cards.

I'll offer a less... apocalyptic take on this card.

All in all, the possibilities aren't mind-boggling, merely intriguing. Like many of the Upgrade cards, it requires you have something you want to attach it to, so you're not going to take it willy-nilly. On top of that, the thing you're stowing needs to be Illicit and take up a Hand or an Accessory slot.

The "vanilla" use case for this card is straightforward: you're playing a Rogue with your hands full, and, in typical Rogue fashion you'd like to squeeze just a little more out of your deck. Say you're Wini with Lockpicks and Mauser C96 out, but a miniboss just spawned and you'd like to legerdemain out a Lupara for that sweet 6 damage turn. Your Hidden Pocket on a Leather Jacket (impeccably maintained by your favorite seamstress) lets you do just that.

A lot of things need to happen before this can work, of course, but I'd argue that a decent number of things need to happen before you find yourself with full hands (and a need to continue to fill them). Upgrade cards are by their nature dependent on other cards hitting the table before them, so it's corresponding a gamble to choose to include them. I think Hidden Pocket's saving grace is that you can let it ride until your slots are full; it's not the kickstart to a combo, it's the flourish at the end to round things out.

Admittedly, I'm a sucker for Upgrade cards, personally, so I'm biased. :)

Let's talk about some potential combos.

At the time of writing, there are 14 (non-signature) level 0 Illicit 1- or 2-Hand slot assets, and all of those are pure Rogue cards except for .18 Derringer ()and .45 Thompson (/).

• I'll lump all the Firearm cards together: You can store an extra gun (or half gun). Handy for Tony, Leo, and Zoey, to name a few popular enemy managers. Finn might like it as well, since you can run him flex and he might have his hands full of Finn's Trusty .38 and 2x Magnifying Glass.
• Lockpicks and Thieves' Kit: both neat Investigate pairings. Sef, Finn, Kymani, and Trish can dig it.
• Switchblade and Knuckleduster: maybe a curious choice if you're not playing Tony, but a weapon with infinite use can come in handy (er, pockety?).
• Damning Testimony: I haven't run DT yet, but it seems good (or at least fun!), so I'll offer proportional hype to be able to hide it in a secret pocket.

I won't say anything about the XP Hand slot cards except that I think the above comments apply to them uniformly. I'll highlight Lupara and Sawed-Off Shotgun as two fun ones to store in a pocket, though.

At the time of writing, there are no cards printed that are Illicit Accessories. With that said, you can always play "I'll take that!" to make any Item card Illicit. In this way, a very charitable interpretation of Hidden Pocket is as a poor man's Relic Hunter.

Simple Armor choices include Leather Jacket (or Leather Coat if you have access). Lonnie Ritter will be happy to tailor these for you, of course, but as others have mentioned, we often run these cards as throwaway soak.

Correspondingly, I think the hype rests with the Clothing assets:

• Robes of Endless Night is usually taken for its economy over its soak, so it's a candidate for an extra pocket if you're Sef or Dexter (or Jim or Jenny, I suppose).
• Track Shoes and Hiking Boots are both great choices. They're slotless themselves and are generally robust to being taken from you (knock on wood). Also, you can attach multiple copies of Hidden Pocket to an asset, so you could conceiveably fit a Chicago Typewriter into your boots. Cheeky!
• Tool Belt is interesting, since it's already affording you a bunch of slots. Kymani likes this combo, obviously, and I suppose you could always Versatile the pocket in to let Joe get an absolutely disgusting number of things on the table.
• Fine Clothes deserve a mention. I usually only take it as teching for Parley-heavy scenarios, but, hey, Clothing is Clothing.

This is the only Dilemma with test icons, though you cannot really hold a Dilemma card in your hand. Regular effects already have a lot of combo potentials and yet it is still waiting for brave investigators to take advantage of this secret commit icons.

Written in the Stars is one card I know can make use of this. (In addition to Darrell Simmons, Vincent Lee also could, since this card heals damage.) Maybe something can commit directly from discard pile after using the 3 + 3 heal option? Maybe something like Astronomical Atlas might come in the future for Survivors?

edit: There is no commit icon on this card!

This card has "Dilemma enabler" written all over it. Certainly the first card I'd upgrade into if I were playing a Dilemma deck. Also 2 stamina and 2 sanity is a great soak. 3 cost and 2 EXP is a moderate cost for the effect, but I don't think you would want to pay it if you have no Dilemmas in your deck. You can view this card as an "additional hand", somewhat safe from hand-milling effects... But it is a very slow second hand then.