I know this is one of those topics that goes around in circles, so I apologize in advance!
My personal play feelings after several years of Sentinels is that the only hero decks that really need some mild buffing are Bunker and Expatriette. I have some Mode houserules for Bunker that I am quite happy with, in terms of being low-intrusion and buffing him up, but I've been going back and forth on my goal of an easy way to houserule Expatriette that keeps the core themes, doesn't require messing with her deck too much, and makes her more reliably fun to play with.
At the moment, I'm juggling two thoughts, and I'm wondering if people have any eyeball ideas about them Both are based in simple revisions to Ammo mechanics.
Thought #1 - When you play an Ammo card, draw a card.
Technically, this revises eight cards, but it's the same revision for each one and it's a really simple one. It upgrades each of the ammos to be a power-tied One-Shot but also to not clog up the deck when drawn, since you get a new card as soon as you play them. One of Expat's big problems is that if she uses her power for card play setup, she doesn't have any native card draw and relies entirely on the support of other heroes. This helps offset that without making ammo too powerful. It also indirectly powers up Speed Loading, by making it so that the ammo Speed Loading sticks on your deck gets drawn if you play an Ammo with it.
Thought #2 - When you destroy an Ammo card, draw a card.
This version has advantages and disadvantages to the above. Mainly, it lets Expatriette draw a raft of cards when she unloads all of her guns, giving her a headstart on her next build-up process. It also means that you get a small advantage if you sacrifice Ammo cards to enemy "destroy equipment" effects, and might encourage more firing of ammo-loaded guns in order to get more cards for future setup, which seems nice. On the other hand, it's liable to lead to burst drawing. It's a bit less intuitive, but I think I might like it more?