Nothing too fancy, and there’s ideas for it all over the internet (which I plan to heavily borrow from).
First up, I wanted to work on the spell list. It seemed that the class should give buffs to luck and therefore dice rolls, and probably debuffs for enemies.
Given the nature of the setting, I also wanted a sort of scrying or divination idea – either through card readings or cheating at gambling.
So, at level 1, the class gets the cantrip Guidance, which doesn’t count against the cantrip limit. Then the leveled spells, so far, are as follows:
1 – Bane, Bless
2 – Augury, Suggestion
3 – Bestow Curse, Call Lightning
4 – Arcane Eye, Divination
5 – Mislead, Reincarnate
Bane and Bless speak for themselves, they buff and debuff, and Bestow Curse is intended as a bit more of the same.
Suggestion and Arcane Eye are for the gambler in the class, with Augury and Divination as some of the scrying idea, and Mislead sort of felt in character for both.
Call Lightning comes from this post on Reddit, and I liked the idea because, in theory, you can make lightning strike the same place multiple times.
Reincarnation comes from this wikia post, and I like it because of the random potential at a high level.
After that, my ideas so far are Insight proficiency, proficiency in a deck of cards, and probably some version of the Fighter/Samurai ability to give advantage on a roll (probably in return of disadvantage) and the Fighter/Battlemaster manoeuvres Evasive Footwork, Parry and Riposte.
I had an idea to have a pool similar to the Lucky Feat or the Diviner Wizard subclass, whereby you could force a reroll of a d20. The DM could then hold that original number and offer the Warlock their point back in return for changing another roll – make their good roll bad, or make a bad DM attack roll good.
It all need more work. I’d also probably pull a few of the Pact of Cards ideas from that above wikia, as I like the Deadlands/Huckster feel of it.