Thank you for your reply. You make a good point.
I think the difference is that I'd like to make the engine an open source project and get some people who are interested in it involved with development. The game would be an off shoot on the engine and would be done by myself.
But I'll see about prototyping ideas on Unreal to start with as you suggest. At least then I'll be able to see what is going on.
Don't get me wrong, Unreal has many flaws But it does have a good solid implementation of most of the core tech you need to make most games.
If you want to go the engine route, I'd recommend Game Engine Architecture by Jason Gregory....gives a good overview of most stuff, and give some idea of the undertaking.
There are a lot of open source engines around...it will be very tough to persuade people to spend their time and effort contributing to your one, amongst all the far better established ones...and while being Linux only may be a positive in your eyes...the reality is that no-one is interested in an engine that only supports Linux...not when there are swathes of existing engines out there that support multiple platforms, including Linux.
Just trying to be realistic....you're not the first person to make a post like this. The reality is that you won't get any useful contributions, and if you are one of the tiny minority of people that sticks with it, will spend years building an engine, and get burned out/bored of it, before you ever think about making your game.
I may be a little biased as I'm an AI/gameplay programmer, rather than engine