Thanks both of you for answering
1) I'm mostly thinking it's safe (& I have Kaspersky which I trust), it's simply that I've not a precise idea of how this works in the facts I was wondering if someone was supervising, or if it was only few "authorized" people who could contribute, or anybody.
For forking the project myself... Ahah, I don't think I have the skills for that But if I've one suggestion, I'll share it
2) Launcher sounds nice so
3) I know well OpenTTD, and that's why I was asking : I never played the original TTD, and loved OpenTTD, but was always wondering how close to the original game it still is. This kind of project can easily loose "its roots", and I love RCT spirit & gameplay. And even if it'll always be compatible with RCT2, the game design/balance may be different and so vanilla game may be "broken" in a way (a scenario is a bit of a puzzle with constraints). That's the hard balance between improving the game with the open version and staying close to the original.
I really love RCT1 and even if I love RCT2, I don't love it as much as 1. But I must say that after trying OpenRCT2, there are good chances I'll love RCT2 a lot more through it So as long as the game has his original feel and can use vanilla assets, that's really great I'm rediscovering RCT2 thanks to OpenRCT2, and this is really awesome