Star-Lord: legendary outlaw, half-human half-alien hybrid, and the most commonly-known leader of the Guardians of the Galaxy. From his MCU debut to the recent Square Enix game, Star-Lord has become an instantly recognizable member of the team. Star-Lord may lead the Guardians in most of their modern incarnations, but there's no reason he has to. We've already seen what could happen if other characters literally replaced him, such as T'Challa in What If? Or when Kitty Pryde temporarily took up the mantle in the comics, and there are plenty of characters who could take his place without having to take up his alias.
8 Captain Marvel (Carol Danvers)
With her own alien origins and intergalactic adventures, Captain Marvel seems like a great fit for the team and as a Star-Lord replacement. She did join the team briefly as a means of finding herself after preventing an alien invasion on Earth and suffering brain damage. She has also led the Avengers before, although how good she was at it depends on your perspective. Maybe leading a team through space and alien worlds could better help her upgrade her leading abilities. It seems her MCU counterpart could use the company after assisting planets throughout the universe by herself. At least the constant references would be changed from the '80s to the '90s.
7 Iron Man (Tony Stark)
The progenitor of the MCU himself, Tony Stark (AKA Iron Man) has joined the Guardians before, but under the leadership of Star-Lord. Nevertheless, Tony came prepared with an improved suit for deep space travel which included the ability to communicate with other suits on Earth from lightyears away, allowing him to have a number of useful abilities at his disposal. With his intellect and previous experience traveling through space and leading the Avengers, Tony would be a reasonable replacement for Star-Lord. Just don't expect to see an Iron Man-led Guardians team in the MCU anytime soon, outside of a possible future episode of What If?
6 Black Panther (T'Challa)
We've already seen an instance of if T'Challa was Star-Lord, but what if he instead led the Guardians after becoming the Black Panther? We know how impressive his diplomatic and negotiating skills are, going as far as to convince Thanos to stop his genocidal plan with a "good argument". Add the experience of being Black Panther on Earth before any spacefaring adventures, and who knows what other universe-shattering events he could prevent? In the Ta-Nehisi Coates Black Panther comic series, Wakanda even develops an intergalactic empire, so Black Panther joining the Guardians and replacing Star-Lord rather than becoming the new one isn't too bold of an idea.
5 Loki
Star-Lord has a handful of his own trickster-like qualities, so Thor's adopted brother Loki instantly comes to mind as a replacement. Loki has plenty of experience dealing with alien species, shady organizations, and dimension-hopping, something the Guardians are no stranger to either, so he'd fit right in with the team. With the MCU version of Thor joining the Guardians, perhaps we'll see variant Loki interacting with the team and edging his way into a spot on the team.
4 Nomad (Steve Rogers)
Like Iron Man, Captain America's experience leading the Avengers helps make him a decent pick to replace Star-Lord, although if he joined Guardians it would be more fitting if he once again went by Nomad since he'd be wandering through space. As an added benefit, he'd probably get the Guardians in less trouble than Star-Lord would.
3 Doctor Strange
The sorcerer supreme may primarily deal with supernatural threats, but he's no stranger to alien threats either. As the primary protector of Earth from other dimensions, it would be beneficial for him to go out into space to explore and possibly even learn about elements of the mystic arts that he wasn't previously aware of. Strange also has previous experience leading multiple incarnations of the Defenders, so leading a ragtag group of space aliens, especially with all of the powers he has at his disposal, doesn't seem like too difficult a task for him.
2 Spider-Man
Spider-Man has interacted with aliens before, but him leading the Guardians does admittedly sound odd. Captain Universe Spider-Man, which is Peter Parker temporarily bonded with an entity known as the Uni-power, seems like a perfect fit for the team. In addition to enhancing his own abilities, the Uni-power grants him Uni-vision - essentially a cosmically enhanced spider-sense - matter manipulation, and telekinesis. If the MCU is building Spider-Man up to be a potential leader of the Avengers like Iron Man, travelling with the Guardians could lead to him gaining an MCU version of the Uni-power and giving Star-Lord a run for his money.
1 Howard the Duck
Howard the Duck may seem like a silly pick, but he's more powerful than you might expect. In addition to having some handy weaponry at his disposal, Howard also has superhuman speed and reflexes, super-fast learning ability, genius-level intellect, mastery of quack fu and the mystic arts, as well as being able to survive in space without oxygen. If Howard the Duck comes to the MCU and has the same abilities as his comic counterpart, we're already voting him as the next leader of the Guardians.