Neon Prime isn't here yet, and there's nothing actually confirmed about it, but data miner and leaker Tyler McVicker gave a fresh overview on what to expect.
Back in October, we found out about the Neon Prime trademark about something new coming from Valve but it wasn't clear at the time exactly what it would be. Valve has stayed completely silent on it of course as well. While Valve of today is far more open than it once was, they're not going to comment on anything not yet announced.
From data mining in Dota 2 updates and other sources, Tyler McVicker mentions it's going to be a third-person shooter with various humanoid characters and has some form of tower defence involved too. You'll even be able to run around and gather resources and it's close to release apart from various bits of polishing work.
You can hear Tyler McVicker's thoughts in the below video. Again though, until it's confirmed by Valve, it's all subject to change and nothing is actually confirmed. It's all just rumours and McVicker has been wrong a number of times in the past but has also been right on the mark a great many times too:
Being a third-person shooter is actually smart. It's part of why the likes of Fortnite are so popular. It's one thing to give you skins like CS:GO, it's another for people to actually see their own skin while running around — and people who buy their skins love that. I wouldn't be surprised if a skin shop would be part of the plan for it with a free to play release.