Throughout the 23 movies that make up the Marvel Cinematic Universe, there have been literally dozens of incredible heroes introduced to us. Almost countless characters came together for the conclusion of the Infinity Saga to take on Thanos, and it was simply epic to see so many joining Captain America on the battlefield.
Though there were too many heroes to count fighting side by side with the Avengers against The Mad Titan, many of them were never officially welcomed to the team, and there were many great characters that never even shared the screen with Earth's Mightiest Heroes at all during their time in the MCU.
The Avengers may be the flagship team of the MCU, and the name on the two biggest movies in the franchise's history, but there are plenty of incredible, strong heroes that have operated outside of the command of Cap and Iron Man.
Whether they were on a different planet, died before ever getting the chance to join the team, or simply fought with the Avengers as something of a one off occasion, there is no reason to think any less of these characters. You don't have to join the team to be one of Earth's mightiest heroes.
Loki has had a complicated history in the MCU, but has enjoyed one of the most incredible redemption arcs across his almost ten years in the franchise. Though he may have started a villain, and acted as the main antagonist for the Avengers' first team up, but he was most certainly a hero by the time he died at the hands of Thanos.
Unlike his brother, Loki's powers never really lent themselves to strength. The God of Mischief instead was blessed with abilities more centred around deception and manipulation. This doesn't mean he is not strong, however.
As a Frost Giant, Loki is almost automatically stronger and more durable than any human in the MCU, and has survived what would have killed most people. He has fought Thor multiple times and lived to tell the tale, drifted through space and didn't die, and even survived the most legendary Hulk smash ever.
On top of all this, Loki beat Captain America in a one on one battle, only losing once Iron Man entered the ring. If Steve Rogers says you pack a wallop, you must be strong!