I think your best bet would be to build an entirely new game with original content and story and make it f2p with a store. You could say it's "inspired by ETQW" and call it a "spiritual successor" if you like.
One question is, what level of success are you looking for? A lot of people do not play totally free, ,open source games that you only find on sourceforge. They don't even go looking for them. They actually expect to pay for their games, and for that you need to be in the payware markets like Steam, etc, and that means you can't take any actual content from ETQW.
Having the game in the payware markets, even if it's f2p, will give you access to the largest audience and ensure the best chance of full servers and longevity.
On the other hand, if you're just wanting a casual replacement for veteran ETQW players, then it might be extremely tough attracting them to your game as a little advertised open source project.
Either way, I would strongly recommend that you have quality bots in your game to keep servers full for off hours and in case the audience turns out to be small. Bots are really crucial, and helped a lot with ETQW.
There's nothing wrong at all with just giving it a try and having a practice project, I just wanted to understand better what your ultimate objective was in terms of success and target audience, because it is so easy to fail in this business, and building an ETQW clone will be epic work.
Either way, I wish you the best of luck, it could turn out really great!
The level of success im looking for is not high, if theres too much people on the servers then the servers may broke, but well, im not 100% sure if the servers will be bad, so I dont really know. Anyways the game will be free and open source so I dont get sued. I dont think releasing it on payware markets.
Yeah, I will try that, I will try to make good bots and not stupid bots.
I dont fail so easily, I have some experience making games, anyways its my first time using Unreal Engine (which some people told me that is the best engine), and when looking on the engine, its very good on graphics.