There are two ways you can go with things like this. The easiest way to go is that the bonuses/unlocks/items/whatever are purely cosmetic. Ideas which come to mind are obviously clothing styles and colors, but also hair styles and colors, entire new characters, other character features, jewellery, pets, design of the pack of cards, design of the chips, even the background/setting, chairs, table, etc. You could also introduce character animations, like taunts, threats, celebrations, expression, and so forth. This is the easiest way to introduce upgrades like this since they don't actually change the game in anyway.

As soon as you start introducing actual game changing bonuses, like having to pay less in to the blinds or starting with more chips, then you start to make your game less attractive to newcomers the longer that it has been active. As time goes on, there will be a small number of frequent users who can collect multiple upgrades/bonuses and give themselves a huge advantage over newcomers. If you do go down this path, then one way to limit that effect would be to create "level requirements" for various upgrades (so to wear the armor that gives +10% starting chips you need to be at least level 5), and then introduce level restricted tables.

In terms of NPCs that you mentioned, it would be cool to have "team challenges". I'm thinking like a 5 person table where 4 people team up to defeat a boss NPC who starts with 10x the chips of everyone else. The team members need to talk to each other and figure out who they think will have the best hand at each stage of the round (hole cards, flop, turn, river) so they know who should call/raise and who should fold to save their chips. Perhaps with extra bonuses like if you get three of a kind you cast a fireball, and so on.

All in all, this seems like a cool idea.