I think it would be better if you focused on this game instead of making a sequel, it is one of the best puzzle games I've played recently. You could consider releasing it on steam or kickstarter. About multiplayer, you could make it simple by making a "play with 2 heroes mode" and make heroes unable to help each other. Minibosses could be a normal sized monsters with health similar to Archeos or prisons.
I must say this is tempting but I already spend 5 month instead of the 2 I was planning for this game. I really need to check if people might like it before I invest more time. Time to check if the game is sustainable and If I have good results ( need to sell >500 units for that ) I may work on a free extension, like the Faster Than Light team did for people who bought their game.