Unvanquished is an open-source FPS game featuring team-based strategy and RTS elements, pitting humans against aliens in a futuristic setting.
https://forums.unvanquished.net/
idtech3 has many variant, such as ET:XreaL, spearmint, darkplaces (Xonotic), and daemon (Unvanquished).
I wonder, technically, which engine is a fusion of quake3, RTOW series, Star Wars series, I mean, if you're looking into game engines, which one is the most worth learning?
The only right answer is daemon, cause this is the unvanquishe forum. 
ET:Xreal is part of the daemon family tree
illwieckz wrote an extensive introduction about the engine for the 0.52 release
Spearmint development has stopped since 2018 with no plans for continuation.
darkplaces had development also stopped for around 7 years but since last year they have some activity again.
The reason is adequate 
The author of ET:XreaL is seems to working on RBDoom3BFG, it is really a amazing and ambitious project!
What do you think? In the furthure, the Daemon engine can compared with that?
Our goal is to make Daemon the best engine that we can make it, but it's unlikely we will reach parity with even RBDoom3 or modern games in terms of rendering and graphics. We hope to have other features that can become differentiators like our usage of NaCl for gamelogic to allow for a portable gamelogic format.