Some thoughts:
- Definitely a dice store. Cover the standard polyhedrals (d4, d6, d8, d10, d12, and d20) at the very least. You can conscript a Chinese plastic molder to make you something competent cheaply, or just license out somebody like WizKids or Chessex.
- Shirts are a must. I think you can get good traction from both the tabletop and digital markets. D&D geeks love to wear D&D themed shirts for some reason
- Customizable DM screen foldout with little clear pockets on the inside for the DM To insert game-appropriate rules summaries into, while the outside features ludicrous over the top “PRAY TO RNGESUS” artwork
- Custom playmats for MtG players, dice towers/rollers for tabletop nerds, and cardsleeves for both CCG nuts and boardgames fans. All with RNGesus artwork as above
- Run a weekly Twitch RPG stream in the vein of Acquisitions Inc. or Critical Role, except that the ruleset is a fully randomized hexcrawl campaign in the vein of what @milspec is plotting over in the Tabletop RPGs thread. I volunteer to DM this for a cut of only 95% of the advertising fees.
- Retire even more wealthy than you were going to already, buried in a gleaming chrome pyramid made from melted down videocard vent covers that your army of hired hands snipped for you