I started to do a FPS game alone and lucky me I found a team, coding a FPS game is not hard at all if you want to invest time on it however doing 3d models, textures, animations and such is what takes a lot of time.
Imho, that all depends on your programming skills. I for myself had no way of doing anything here 4 years ago… but I’m programming for 3 1/2 years now and decided to jump into unity again 2 weeks ago, and yeah… pretty much straight forward, capable to do complex kinda complex stuff already. So if you are well known with C# and know your grips, just go with any idea you have… you’ll probably find a way to do it properly, as there are some crazy things you can do!
And especially for your project, even if you can programm only a little, that will help you developing your skills way further than now, good base to also learn a lot!
Will you be able to make a game like that: erm… kinda depends on you, and it’ll probably take a little while to build up to a fully featured version, and remember that most games are made by lots of people not one (but sometimes only one)…
Best thing in my opinion is to try to make something, anything, learn how to work inside the unity framework. Pick a couple of mechanics from the games you really want to make and try to build them into a simplified version.
I agree with LeftyRighty; just keep making stuff. Even if you drop the project before finishing, at least you’ll pick up a lot of skills.
If you have the budget, I strongly recommend starting with a framework on the Unity Asset Store such as the Hack-n-Slash RPG Kit. It’ll get you off to a quick start with something immediately playable, and you can pick apart how other developers implemented certain features.