Well… can't be worse than Diablo 3.
… or can it?
1. Release act 1 as the full game then sell the subsequent acts as expansions.
2. Microtransaction lootboxes that contain things borderline essential to advancement, such as abilities and spells and they can't be obtained from drops.
3. Class-specific gear never matches the class you're playing, forcing you to trade with other players.
4. "Energy" system like you see in many Facebook and mobile games, limiting the amount of time you can play before you have to purchase more energy or wait for it to recharge. Linked to account instead of individual characters.
5. VIP subscription that speeds up the energy recharge and makes it so items better than white and grey quality can drop. Also includes access to many quality-of-life features that are standard in most online games these days.
6. Overhaul the artistic style of the game completely and go for a colorful, super-deformed anime style.
7. Make grouping forced by making the content literally too difficult to solo. But only give access to matchmaking and group-finding services to VIP subscribers.
8. You get one (randomly decided) class made available to you when you buy the game. The other classes (and additional character slots) can only be acquired via premium lootboxes.
Alternatively, just go the Call of Duty route and release Diablo 3 as Diablo 4 with marginally better graphics and a slightly different story.