Ah alright. I'm going to call and talk to a local law firm and see how much it would cost

I really think you should contact an attorney with experience in the game industry. See the stickied list on this forum.

It's unclear if you are planning a commercial project with people (including paying for their contributions), or are just doing this for fun with making money an optional extra later on? Similarly, it's unclear if this is talking about releasing as a private repository, or a public one? Is this just to share with people working on it, or do you expect random people to start using it in their games?

A commercial project is better done with lawyers and so on - and doesn't need to be publicly hosted. But for the latter, I'd argue that using an Open Source licence is better:

"The engine requires a lincse to use commerically which can be negotiated with me."

There is no Free/Open Source licence that will include this part (by definition), so the GPL isn't useful for you.

But I agree with Servant of the Lord here. My advice would be to consider whether you really need that clause.

GPL would in practice deter companies from using it in games (due to the requirement to also licence their game code as GPL), but in any case, is a commercial company going to use your engine? If they wanted to use a free engine, there are already plenty of high quality Open Source engines out there, including those under licences that would allow them to keep their code closed source (e.g., LGPL, BSD). And most either pay for commercial engines (or write their own), AFAIK.

You also have the option to dual licence, e.g., under GPL, and a paid-for licence that allows use with closed source (e.g., Qt is licenced under LGPL, and a closed source commercial licence).

A reason why Open Source licences allow commercial usage is that it avoids a lot of gray areas. Can someone put it on a website with adverts? Can it be distributed on a Magazine CoverCD?

Using an Open Source licence means you don't need a lawyer to write the licence. I'd also argue people are more likely to contribute if it's something being done for free, as well as more people using it, due to the clearer more well understood licence. True, CC BY-NC would do what you want - but remember the resulting engine would be incompatible with Open Source, which may not be ideal if you want wider support from people working for free.

And it also means that you can use hosting sites, including GitHub, for free (your custom licence means you could only host on GitHub with a paid account, I believe).

SimonForsman, on 26 Jun 2013 - 2:50 PM, said:
"To me that sounds as if he wants full, non exclusive rights to contributions made for the "free" version so that he can include those in the commercial license without having to pay the copyrightholder for a separate license. (This makes most existing opensource licenses unsuitable)"

Also consider, why would someone want to give him full exclusive rights to their contributions, including commercial usage, when they (or anyone else) doesn't get those same rights? I mean, I'd happily contribute to GPL, BSD or whatever else, knowing that my changes are released to everyone under that same licence. I'm not sure why I would work on someone else's non-Open commercial project.