Loki is not a Background Character

If you didn’t have Loki then the Avengers never would have assembled to defeat him in New York. 

If you didn’t have Loki then Ultron would never have been created, The Vision never would have been created by Ultron, and Wanda Maximoff and Pietro never would have gained their powers. 

If you didn’t have Loki “fail” to bring back the two infinity stones to Thanos then it wouldn’t have been as important and necessary for Thanos to quickly get his hands on another stone so Gamora may have never been sent and she may have never met Quill, Drax, Rocket, or Groot under the circumstances that she did. Ronan picked her to send because he presumably knew Thanos really needed the stone after losing 2, and Gamora was better than her sister. So the Guardians potentially wouldn’t have happened. 

If Loki didn’t take over and pretend to be Odin he would never had the chance to split up the Infinity Stones in TDW and ensure they wouldn’t be easy pickings for Thanos. 

Many of the major events in the MCU can be tied back to Thor 1 and Loki falling into the void. 

Loki was the catalyst for the mcu

Loki was the catalyst for the mcu, and that's one reason why IW was such a disappointment for me, as the conclusion to this story that had been built for a decade.

Loki was the catalyst for everything, and we'd been first introduced to the big bad of IW through what he did to Loki. If the Russos wanted to establish Thanos as super evil and ruthless straight away, (or... trying to save children from starvation while also relishing in bloodshed?) I feel the best way to have done this would have been to clarify the real story behind the first Avengers.

The GA sees Loki as the bad guy of that film. Show them he was just the puppet. A puppet with enough strategic intelligence to foil his master and cut his strings in the end, but a puppet who was still forced to dance to his master's tune and go through the motions all the same. Because as powerful and terrible as Loki is, he was dominated by this being. It took all the Avengers to defeat Loki (and whether they actually defeated him is even debatable), but that power is nothing compared to his puppetmaster. This being made a GOD into his puppet. Isn't that a truly terrifying prospect?

And look at what Thanos was trying to achieve through Loki. He wanted the Tesseract, yes, but if that was all he wanted, Loki would have just gone straight back, successful, after the first 5 minutes of the film. But he also wanted Loki to create a portal through which to send what looked like a pretty darn massive army. The only purpose of that army was to destroy -they started as soon as they came through, without any direction, and if the entire army had gotten through the portal, the damage would have been pretty extensive. So Thanos is a guy who tethered a god, gambled the mind stone, and assembled thousands of Chitauri and their tech/leviathans/weapons simply with the intent to cause as much destruction as possible.

Reframing the first Avengers with that intent revealed, that this mad titan will go to insane lengths to just kill and destroy everything, makes Thanos a lot scarier and more compelling (imo) than anything we got out of IW. In addition to introducing Thanos as this terrifying entity, it would have begun the film with an actual conclusion to the plot threads left hanging from the first Avengers.

And Loki is absolutely essential to this story. The characters need him in order to understand or realise any of this. Bruce magically knowing Thanos and his plans does not cut it. Loki's is the frame of reference needed to warn the Avengers of what is coming, what Thanos plans, and the extent of his power. Loki is the one who brought them together, for that very purpose. Loki is the one who split up the space stone and reality stone to keep them from Thanos.

Loki was ruler of Asgard for the past 4 years, with all kinds of magic artefacts and resources at his disposal. Loki would have enacted multiple plans to either protect against Thanos or to defeat him. Loki was told that Thanos was coming for him, that there was nowhere he could hide, and he would long for something as sweet as pain when they got to him. Loki was terrified. Loki WOULD HAVE planned against that. Loki should have been heavily involved in IW, helping the Avengers and using everything he had planned and prepared for this day.

LOKI SHOULD HAVE BEEN A MAJOR CHARACTER IN IW. He should have been the Avengers' main asset against Thanos. Loki is ESSENTIAL to the MCU story arc.