2Son of the magus killer idea
I own nothing expect this idea
Inspired by "the story of Illya Emiya" by LunarSkies2383
Second try, please don't flame
In many visions of the Kaleidoscope, the bloodline of Emiya ends in tragedy and death, all due to the distorted ideals of the father who was once the magus killer. However few minor differences can change everything. Here the vessel was born a boy. The child who was saved was a girl. The maids would be made eight years earlier. The boy refused to believe his father had abandoned him and with the maids escaped. Meeting his sister and reunited with father, the boy will become a man. The son of the magus killer.
Basic plot idea: Shirou (I will call her Shirouko) and Illya (I will call him Ivan) are genderbent (you chose different names). Ivan runs away from the Einzberns with earlier made Stella and Leysritt. He reunites with Kritsugu and meets Shirouko, joining the Emiya household. He convinces Kritsugu to spill the beans on the holy grail war (he will tell Shirouko later.) and teach his sister magic. Kritsugu then with Stella's and Leysritt's help, creates a body for Ivan with a normal life expectation. The rest is up to the author.
Ivan's personality: Ivan is like Illya and Irisviel in pubic: playful, friendly, childlike and a siscon. Hover in combat and moonlight world situations, he becomes like Kritsugu: seeming cold, cautious and crazy prepared. However he has learned from his father's mistakes and will trust and comfort others and WILL NOT take the policy of sacrificing the few to save the many.
Note: if Arturia is summoned, display her reaction to Ivan and her views on he's inherited personality traits.
Suggestions: have Shirouko go though character development due to gender being different and Ivan's presence.
Explore Ivan's and the maid's relationship. In canon, Stella and Leysritt are the closest thing to actual family (besides berserker) Illya has after the fourth war. No one explores this.
If you're going to use fate/ kaleid characters, try to do them like this:
Kuro/Chole: separated twin of Ivan, complete clone of canon Illya expect for yellow eyes. No reason for tanned skin.
Miyu: the Ainsworths grey grail, created from stolen notes from the Tohsaka. (Always wondered why they never try to chest the system, they helped create it, lack of pressure maybe?) However if you do this, you may have to increase the servant numbers.
The Ainsworths: want the grail to "save the world". Julian may be a major antagonist. The dolls are homunculi like Stella and Leyrsitt, designed to subdue/gain control over/defeat servants.
Card servants: greyed servants. Like blackened servants, these are servants corrupted by Miyu if she turns "dark". However if black represents despair, grey represents greed. Grey servants are like card servents expect much stronger, retains more intelligence and a different appearance.
Ivan could find a way to "whiten" servants into "lilies". White represents good/hope.
If Sakura still likes/wants Shirouko and she turns dark, she and Ivan could fight an epic battle over her (apparently they are natural enemies). Better if Sakura is genderbent.
Possible lover/harem list
Shirouko: siscon Ivan. Enough said
Saber: knight and price pairing
Rin: humane magus pairing
Sakura: Ivan could save her instead of Shirouko
Medea: same as Sakura
Medusa: extra package with Sakura
Stella and Leysritt: maids and master pairing
Shielder: female Galahad, original idea in progress
Reikia: yandere.
Taiga: Tsundere pairing
Miyu: vessel paring
Dolls: new purpose pairing
Ayaka: prince admiration paring
Manaka: yandere with dream of threesome (could be another vessel with her own "colour")
Elsa: motherly pairing
Misaya: cold magus paring
Luvia: noble pairing
Kuro: incest pairing
Bazett: saviour pairing
Fiore: kind princess and selfish prince pairing
Run Ru: fixing pairing
Kiara: lust paring( also could be a vessel with a "lust colour")
Rani: homunculus pairing
Alice (older): playtime pairing
Other than that, go wild. Go "war of roses", introduce more vessels, I like said before, I didn't care.