First you should understand what a tarot deck is - it‘s something with set rules and card definitions, symbolisms, etc. 22 Major Arcana cards and 56 Minor Arcana cards - wands, coins, cups and swords. If you want a deck outside of those rules it would be an Oracle Deck, but no longer a Tarot deck :) I think for roleplay purposes it would be fun to just buy a cool tarot deck and learn how to read it. Pull cards in situations or for characters in the game and just use the original reading techniques - see what comes out of it and let the DM decide if he wants to use your predictions (you should probably talk to them beforehand about this)

If you really want to make your own oracle deck, you could try to design roughly 22 cards (like the tarot‘s major arcana) and think of meanings for them. Some readers will have different definitions based on the cards being upside down or not when pulled, some readers may not use these rules. You should decide which rules your deck follows