The overworld can be done entirely solo with a storyline for each of the three factions, but you can also partner up with friends ad hoc if you'd like.
The content that's specifically multiplayer is the dungeons (4-man or 12-man), but you can queue for the former and people will zone chat to organise the latter
Use the maximum HP possible from the dice instead of the average given (eg. 6d12 = 72 instead of 39), or at least a higher portion of the maximum quantity
Increase AC
Give it extra damage of a different type
Give non-lair monsters lair actions, and give monsters with lair actions an even stronger lair action they can use when below half-health. Same with legendary actions
Look at older DnD editions and see if the monster or any similar monsters have extra abilities you can add
Edit: I should have specified that these are in ascending levels of difficulty for the DM, but are also more interesting