Adding Multiplayer Features isn't that bad, infact, make the game even more fun rather than solo (Max 2 Players)
Here's what the idea
Boss Damage and Health increase by x2
Enemies Damage and Health increase by x2
More scrolls to pick
More Enemies
Chest Rate Doubled (Normal and Cursed) (Bcoz It's multiplayer)
Bigger Maps
If a person died, they'll respawn on the passage as long as the other player still alive
Difficulty adjustment (If the hosted player is currently on 5bc, lower player automatically gets 5bc on multiplayer, same goes to lower player hosts)
And here's how's the multiplayer works
First person create a room with a code written in the beginning of the area, and then the other players type the codes (5 letters code max)
Since this game is on several platform, it's better to make the multiplayer server cross servers so any players can join a room
I think this is only my suggestion