Zelretch was bored. Seeing as Shirou Emyia is usually one of the more fun targets of amusement, it's only fitting that he should mess with him. But what would be more interesting than pulling mere pranks on him.

"I know! I'll merge all those bad end Shirous into one and send them somewhere he'll be sure to put it to use." Zelretch smiles mischievously.

Now where and when to send him? Back time seems interesting. Maybe put him into a self that is about to start the 5th Holy Grail war. Or perhaps during the 4th war. Make him reborn as that Sieg homunculus or Shiki Tohno. Make him the master of Chaldea. Put him in Prisma. Turn him into a high school student of Servant High. Or should I put him into a more remote dimension. The possibilities are endless! Screw it, I'll just do all the above.

Which dimension would benefit more(most fun to see) with Shirou as the MC. AOT seems like it would be hilarious him punch, slash, and toss titans around. MHA would also be a good choice. UBW plus quirks equals one punch man. That one is also a good one. The shield hero, more like unlimited hero. He would be the perfect red dragon emperor. Making him Bell Crannel would also be fun.

Maybe a mecha world. The perfect coordinator, more like the ultimate. The prince of Britannia would also be. He would do well with a IS or hundred. The weakest undefeated, is actually the strongest undefeated. Destroying an Object barehanded seem good too.

The pirate king seems far too amusing. Making him an Uzamaki or Kurosaki seems far too good to pass. The white knight of Remnant has a nice ring to it. King of the galaxy sounds good to me. Make him the tragic loner of SNAFU or put him in a class of the elite. The fire dragon slayer with unlimited weapons. The greatest combat butler to ever live. Putting him in Dessert Punk would too fun not to see. Night Raid can just sip tea while Shirou wreaks havoc on corruption. Stuck in SAO for years, never fear for Shirou is here!

There are way too many good isekai to choose from. Wielder of the ominitrix maybe. He would make a great super boy as well.

"Shirou Emiya find dear friend, you are just way too much fun to mess with. This is why you're my favorite person to watch. I'll even buff you with some perks".