I've had a lot of ideas for other classes because I may eventually add subclasses/class specializations down the road. Each class would have 3 classes to specialize into. Summoner and shadow knight were ideas that I also had for other classes. The problem with summoner is that it would be a bit difficult to implement it the way I'd want, most other classes wouldn't be too big a deal to add. Engineer could fit into the world (as in it would exist in-universe) I'm just not sure how it would work, I'd have to think about it more but I'll add it to the possible future classes.
For how long it will take to complete, I gave a best case scenario that it would take like 16 years or something if I continued to work on it by myself. I knew from the beginning that this would be a very long term project but now I don't know if it will ever be complete with how much I keep wanting to add and all the details that I want. I don't want this to be a game that is never finished so I'm reconsidering what the priorities should be and where I should focus.
I wouldn't say I prefer to make the game myself, I just don't know anyone else or trust anyone else enough to work on the game with. Plus, I don't think anyone is going to want to work on the game for free like I am. I'd love to have a small team to work on this with, it would make my job much easier and I wouldn't have to keep splitting my focus across the different aspects of the game. Maybe if the game gets popular enough or I start making money on it I can get a small team to help out. Until then, I'll try to focus on adding what's important to keep the updates coming.