Not gonna lie, I really wish you'd at least finish their fights.

I absolutely love this. You should try a character from RWBY or Captain America.

Caster Alucard - Anime Hellsing vs Karna

Next, you should do Kellam from Fire Emblem Awakening. With EX Presence Concealment that he can't control.

Or Saber from Fire Emblem Echoes. Just to mess with Kiritsugu with a "Who's on first" joke.

Think you forgot another one of his Noble Phantasms: He's a right cunt-A; John Constantine, through his truly legendary acts of dickery, is guarenteed, if using his Phantasm, to out wit anyone that can not match an intelligence check of equal level to his Phantasm.

I think spidey would make a good servant class Ruler.
He does inspire people and lead an army of spidermen.

Try Undead Ban (the Fox's Sin) from the Seven Deadly Sins (Nanatsu no Taizai).
He is an unkillable power vampire - how frustrated Gilgamesh or Lancer would be with an opponent who just. Can't. Die!
Or what about Archer Emiya trying to trace his powers and getting them stolen instead!

Pokémon

Do the Pokémon Platinum girl.
You know, with the Sinnoh legendaries.
Which, well, basically gives her indirect access to the Kaleidoscope.

Reality Marbles as Noble Phantasms don't really work...
Rather, weapons that have such an effect or rather items in general tend to be how they work.
Or possibly just standardized Bounded Fields that end up rewriting reality to an extent.

Even still, Holmes is under the Ruler class, despite carrying Caster-like Class Skills - and with a "Revelation on steroids" Noble Phantasm.

Okay, at first I thought it was Ash. But...the silence. It spoke of one who was his progenitor.

Recommend listening to Seigi Shikkou here

Heya!
Re-read these shorts of awesomeness, and (of course) came up with a new suggestion. Most likely a Saber, though, to be fair, Caster also works and Assassin is doable in a pinch. Now, the Grail Wars have a pletora of Heroes to chose from. Some might even argue...to many. Perhaps what the Wars need is not another Hero at all...
...but a professional.

Geralt of Rivia. Witcher. The White Wolf.
Who better to fight the Corruption of the Grail?

My suggestion?

Miss Piggy as Berserker.
Just one comment about her weight or wardrobe, and she can karate chop the fourth wall!

how about we get some original dragon ball with eighter the Android

It would be pretty amusing if Default from watch dogs showed up as caster or something, the grail war is set in a city, which would give him a immediate advantage, as he is a hacker and is surrounded by electronics.