In the words of the dev: "
Time Tripper is a freewheeling first-person action-adventure-STG that runs on the GZDoom engine."
The biggest change in Time Tripper from regular Doom is that, excepting the Arch-Vile, enemies no longer have any hitscan attacks. Even the Zombiemen, Shotgun Guys and Heavy Weapon Dudes now fire projectiles that can be dodged. Which is important because Time Tripper has a one-hit kill mode—where both the player and the enemies die in one hit from anything. I played through that mode yesterday, after having beaten the UV-equivalent regular difficulty the day before. And the one-hit-kill mode of what could basically be considered the "second loop" difficulty today. It really does make it feel like a blend of Doom and a shmup/run'n'gun. There's even built-in checkpoints in the stages.
Existing enemy projectiles have also been changed, and some projectile graphics are directly taken from stuff like DoDonPachi. Think you know how to dodge a Cacodemon plasma ball? Now it's a barrage. The Hell Knight now fires a wide spread. There's also a whole lot of brand new enemies.
The visuals are great. The soundtrack rules and is unlike anything I've ever heard in a Doom WAD before.
I'd post screenshots but tbh stills don't make this game justice, it has to be experienced in motion.
So it's a game I really can't recommend enough if you want a game that's Doom but also arcade-style shmup/run'n'gun action. Which seems like it would be to the tastes of a lot of people here.