This would be really controversial and quite possibly impossible to pull off but I’ve always imagined an ARPG-strategy-survival-single player MOBA hybrid game.
The ultimate goal would be to kill Diablo but you start out and you can go anywhere on the map instead of proceeding linearly through act 1 to act 2 to act 3 etc. There would be a bunch of optional side quests that have time limits on them and you have to complete them in time to receive a significant benefit that will ensure the survival of the town. You save the town, you might unlock mercenaries, or craftsmen, or women who can cook that you can recruit to help save/defend other towns and ultimately help in a massive battle like D3 act 3 first part but more epic, Helm’s deep epic. Towns would be periodically attacked by enemies once you completed the initial “save” quests. You don’t necessarily participate in those battles but the towns survival depends on how much help you provided and you can send some of the mercenaries there to help defend, nurses and cooks to heal and restore moral, to ensure the town doesn’t fall to the next attack so that you can continue with the benefits that town grants you.
The idea is that you have to save several towns and doing so is crucial to success in the main story line, but you can’t save all the towns.