Premieres December 31, 2024. The new PV is here. Check out the ANN article for additional information. Synopsis from AniList:
In a Holy Grail War, Mages (Masters) and their Heroic Spirits (Servants) fight for the control of the Holy Grail—an omnipotent wish-granting device said to fulfill any desire. Years have passed since the end of the Fifth Holy Grail War in Japan. Now, signs portend the emergence of a new Holy Grail in the western American city of Snowfield. Sure enough, Masters and Servants begin to gather...
A missing Servant class...
Impossible Servant summonings...
A nation shrouded in secrecy...
And a city created as a battleground.
In the face of such irregularities, the Holy Grail War is twisted and driven into the depth of madness. Let the curtain rise on a masquerade of humans and heroes, made to dance upon the stage of a false Holy Grail. This is a Holy Grail War covered in lies.
Honestly, people make it out to be harder than it is. There are 3 mainline routes: Fate (the Deen anime), Unlimited Bladeworks and Heaven's Feel (both ufotable). These are the ones that actually matter.
Regarding the order:
That's it.
Zero is generally recommended as well, because it's a really really good prequel. It can be watched before or after those 3, depending on who you ask (I watched it before and think I prefer it this way, but eh, opinions differ). 1st episode is very exposition heavy, but after that it's a great time.
All the other stuff is all based on the concept of these three routes, but aren't directly related. You can not watch it, and not miss anything. If you do want to watch the others, you do need the basis of the 3 routes, but they're all standalone from each other.
Most of the discussion is just opinion and centred around Zero and the Deen adaptation for a multitude of reasons, but its basis is not at all difficult or messy.