I’m already a part of a team building a MMORPG in Unity3D. But that is just my day job.
After playing around with the unreal engine, I’ve really started to like the tools sets and overall workflow. What I really aspire is to make games as an Indie Dev. I want to make games which are great by experience.
As far as the type of game is concerned, that totally depends on the outcomes of prototyping and what gameplay is achievable as a good expereince. (Most probably turn-based or session based. Definitely NOT an MMO. 
That would kind of naive as an indie dev )
The difficulties I feel I’ll face are:
- Maintaining Server.
- Optimizing network calls and synchronization.
- User experience over the network.