In the Unlimited Blade Works scenario, Gilgamesh intervenes in the Fifth Holy Grail War, kills Berserker, and seizes Ilya. If he had not gotten involved, can we guess based on knowledge from other scenarios what might have happened?
Scenario 1: The Einzberns cheated....again! and thus win
the Einzbern family were at one point holders of the 3rd Magic, Heaven's Feel, which is what the Holy Grail War was original for, reclaiming this magic that at some point the Einzberns lost. Zouken had interest in Heaven's Feel himself because of how his soul was rotting and the Tohsaka only sought to use the Grail to reach the Root.
However the Einzberns have had a somewhat unfair advantage, the Greater Grail's "soul" if you will is that of Justeaze Lizrich von Einzbern and the Einzberns have always had the components needed to finish the war.
- the Lesser Grail which origianlly was a cup but after the 3rd war being the form of a Homunculus similar to Justeaze because if damaged would end the war as was the case in one of the wars and one needs the Lesser Grail to use the Greater Grail to have their wish granted
- the Dress of Heaven which can control the Greater Grail
and in different versions of the third Holy Grail War the Einzberns use their knowledge of the System to create a new Servant to get the upper hand, either replacing Beserker with Avenger (which failed horribly) or summoning Ruler to be the absolute authority.
in this answer i explain what i believe the Einzbern's plan was in regards to Ilya. since she's the Lesser Grail if she was killed (being a Master) then no one gets their wish. it's like the Einzberns taking away the ball from the ballgame if things don't go the way they want, but instead of taking the ball away they make every other player destroy it. (it's for this reason Kotomine and Gilgamesh needed Ilya (or at least her heart) for their purposes)
Scenario 2: Angra Mainyu is born and no one wins
in the 3rd war the Einzbern Family summoned Angra Mainyu in the Avenger Class but was quite weak and was killed off early. his death contaminated the Greater Grail.
Kiritsugu concluded after his encounter with Angra Mainyu (talking to him via the radio, TV and appearing as Irisviel) that any wish would lead to destruction, even if Kotomine and Gilgemesh didn't have their own plans and a Master or Servant were able to get a hold of Ilya as the Lesser Grail for their own use and make their wish this would release the six billion curses of the world which Angra Mainyu now embodies.
The materialization of Angra Mainyu brought forth by truly opening the Holy Grail is an ichor that will consume every human being, 6 billion curses flowing out from the gate to hell. Gilgamesh specifically mentions 5.6 billion curses for an unknown reason. It is a reaper that will strike down all humans equally, an endless darkness that is born from humans and massacres them.
in the Fate and Unlimited Blade Works routes of Fate/Stay Night even before anyone makes a wish the mud from the grail is already spilling and corrosive. in Fate when Shirou fought Kotomine he has to take care to avoid being hit my the mud Kotomine shoots at him as at one point he is grazed by it and is almost succumbed to the curses.
in Unlimited Blade Works Saber stops Rin in wading into the Mud Pool to reach the deformed Shinji because of what it would do to her (if i recall it was Saber's abilities which gave her some protection but it was describing that it was eating away it her)
Sakura as the Lesser Grail
while Sakura is also a lesser Grail thanks to Zouken infusing fragments of the one from the 4th war into his Crest Worms that are inside her Sakura's connection to the Greater Grail is to that of Angra Mainyu itself.
before she realized she was The Shadow it was already consuming people and Servants and unleashing Saber-Alter and Blind-Berserker. while Zouken might have been able to control Sakura for years in the end The Shadow wasn't under his control. it would have only been a matter of time before it controlled Sakura as the ribbons that's seen on The Shadow make up Dark Sakura's dress which was then used to bind her after her's and Rin's fight near to the end of Heaven's Feel.
I get the impression Illya would've agreed to team up with Shirou & Rin temporarily (Despite one of her goals being to kill Shirou) Kotomine would've then not order Lancer to help Rin, and waited towards the end of the war before he intervened.
Illya, Rin, Shirou, Berserker would then arrive at the church, to defeat Caster, Archer would then have to take on Berserker 1v1.
This fight could go one of two ways, Beserker could overwhelm him, since Archer has no form of terrain advantage, or Archer could spam noble phantasms(like Gil albeit slightly slower), buy enough time to summon UBW, then sword spam the rest of his lives away(Quicker than Gil). I'm assuming Illya will stay with Berseker though, when Shirou and Rin go to take on Kuzuki and Caster. I'll assume Archer wont sword spam in case he hurts Illya. So i'll just say Berserker kills Archer, although Archer is at the very least taking 6 labors ... Which incidentally will make Shirou useless for the remainder of the route, since he can't gain anymore of his abilites/knowledge, and his projections are poor without understanding UBW.
Rin/Shirou VS Caster/Kuzuki continues as normal, until Berserker bursts in and kills Caster & Kuzuki. At this point Illya probably cares a bit more for Shirou and doesn't want to outright murder him, HOWEVER as a master she'll probably decide this is a good time to take out Saber. Rin will probably also want to seize this opportunity, and will taunt Illya, into allowing her to make a contract with Saber. Saber may object a bit, but she'll see it as the only way to save Shirou and Rin, Shirou will instantly agree to it...
They then agree to fight OUTSIDE of the church (pretty much for the sake of plot, but i'm sure the Berserker/Archer fight, and Berserker breaking into the scene would've caused significant damage to the structure, so it's unfavorable for all combatants) Saber can compete with Beserker since she has a compotent master, holds her own, defeats him with Excaliblast when the oppertunity presents itself. Illya then teams up with Rin, Shirou, Saber.
Illya gets kidnapped by Kotomine from Emiya household. Taken to Fuyuki temple, used as lesser grail (killed). Saber defeats Lancer, Rin & Shirou defeat Kotomine. Then it ends like the Sunny day ending of UBW :)