Ok I just wanted to get my cast down and initial pitch. I will add much more later.
The cosmos has changed significantly and as such the residents of the DC Universe have to change as well. Long gone is the act of planets acting singularly while depending on the Green Lantern Corps for protection. Due to threats such as alien despots and the GLC themselves turning on them as the Manhunters did, several of the largest planets in the universe have banded together to form a group of heroes from different planets and times to protect their home worlds. Taking the name of the universe's greatest champion Superman's idols, the Legion of Superheroes, these new United Planets have tasked Superman with selecting the greatest heroes in the cosmos. These are the greatest adventurers of time and space, as their worlds' future rest on their shoulders. Selected by Superman to enable to well being of the universe the Legion of Superheroes are the best multiversal, time and space explorers in the universe.
Barry Allen, the Flash
Home World- Earth: Hailed as the savior of not only the human race but of the entire Multiverse, Barry Allen is now one of the most influential and legendary heroes in the universe. Barry is given the option of leading the Legion by his old friend Superman; a task he is more than willing to take up if it means exploring the cosmos and ensuring the future is preserved.
The Flash is the best of the best; able to move at speeds approaching the speed of light. He is also the greatest Multiversal explorer in existence, and the progenitor of the great Flash legacy.
Kara Zor-El, Supergirl
Home World- Krypton: Acting in Superman's stead as resident Kryptonian, Kara is given a choice to be the powerhouse that the Legion needs. Joining without surety in her future, Kara will become the future of the Kryptonian race that her cousin intends her to be.
Kara is a full blown Kryptonian, gaining immense strength, speed, flight, and invulnerability and vision powers. She is the most powerful member of the team by far.
Thom Kallor, Starman
Home World- Xanthu: Given a shot to help kick start the team that he eventually joins in the future, Thom already knows how this Legion will end up, and how to prevent its eventual destruction on a dangerous mission.
Though insane, Thom has immense ability, allowing him to alter the density around him. He also has an original Legion Flight Ring, access to history databanks and a map of the entire Multiverse on his suit.
Luornu Durgo, Legion Lass
Home World- Cargg: Though her third duplicate Una died along with the Karate Kid, her original dupe had been kept in stasis by Brainiac 5 in the future. Sent back in time to finish her original mission, Luornu has been brought in to join the Legion due to her all new ability.
Genetically altered by Brainiac 5 in the future, Luornu now can create up to a thousand copies of herself, in a sense becoming a true Legion on her own. She also has access to a Legion Flight Ring, and history banks of the future.
Alanna Strange
Home World- Rann: With madness erupting around Rann, Adam Strange is forced to provide diplomatic stability amongst the people. Requesting member from the planet Rann, Adam's wife Alanna takes up his suit and acts as the resident adventurer for the Legion.
Alanna has all of the abilities that her husband demonstrated as Adam Strange, her advanced technology giving her flight, force fields, ray guns etc. Alanna also acts as the team transportation, using the Zeta Beam to carry the team across the cosmos in seconds.
Katar Hol, Hawkman
Home World- Thanagar: Feeling conflicted over his past after an encounter with a Monitor; Carter Hall has embraced his alien side, becoming Katar once again. Katar now acts the war hound of the Legion, leading the team tactically on the battlefield and flying face-first into a flight. Katar will discover his true destiny no matter what it takes, and the Legion is a means to this.
Using the advanced Nth metal Katar can fly, has immense strength and speed. He is a master combatant and strategist.
Vril Dox, Brainiac 2.0
Home World- Colu: Leaving LEGION to join the Legion of Superheroes, Vril is promised almost unlimited scientific pursuit in exchange for his expertise. Coming into conflict with Supergirl based on his predecessor, Vril becomes even more confrontational than usual.
A 12th level intellect, Vril is the greatest scientist that the Legion could afford to get.
Salakk, Green Lantern 1418
Home World- Slyggia: Though the Legion of Superheroes act without the edict of the Guardians of the Universe, the GLC still demands that their operations be monitored by a member of the Corps that they could trust. Choosing Salakk, the most trustworthy of the Lanterns, the Guardians have their ear in the Legion's business. Salakk acts as internal affairs, making sure the Legion acts under a strict moral code and in clockwork fashion.
Salakk is one of the greatest of the Green Lanterns, and is unparalleled with ring slinging. He is a strict and upright worker and is almost flawless in his decision making.
Arc One, Whatever Happened to the Legion of Superheroes? : The United Planets are formed under the code of Truth, Justice and the Universal way. At the behest of Superman, they choose to look to the future and create the first generation of the Legion of Superheroes. The greatest cosmic explorers are selected and form under the banner of the Legion. Barry Allen is selected as leader of the group and is given the task to lead them. The Legion at first bickers due to cultural and emotional differences, but are nonetheless called upon to fix a deteriorating rift in space near Rann. The Legion attempts to quell a rift in the universe that threatens to pull the positive matter universe into the antimatter universe. Finding the culprit of this to be Mr. Mxyzptlk, who has fulfilled his prophecy to only commit acts of evil for the next 2000 years as written out by Alan Moore in "Whatever Happened to the Man of Tomorrow?" the Legion chases him on a tour through time and space. After defeating Mxyzptlk by throwing him into the Phantom Zone the Legion winds up in the 41st Century.
Arc Two, the Legion of Superheroes in the 41st Century: The Legion of the 21st Century finds themselves in the time period of the Legion of the 41st Century. They interact with the decendants of the 31st Century Legion, including Cosmos Man, Lightning Woman, Trillicate Woman, the Phantasm, Solaris, the Master, Brainiac 52, and Magnanimous Man. The Legion attempts to find a way home with the help of Brainiac 52, but are betrayed by Solaris, who was corrupted and taken over by the Tyrant Sun. The Legion is overtaken by Solaris, until his death at the hands of Magnanimous Man, the former Ferro Lad who absorbed the Sun Eater and uses it to absorb Solaris. This would signal Ferro Lad's eventual death once again and force the Legion to retry and find a way to get home.
Eventual arcs would see the Legion traveling the Multiverse seeking to find their way back home. I'd explain more but this about the cast not the story arcs.
Creative Team Well I would definetly say Keith Giffen just for how well he handles cosmic teams, though I would freaking kill to write this comic and have thought up enough stories to fill up seven story arcs at least. I'd have Gabrielle Del'Otto do the redesigns for these guys and their new space duds, and have him do interiors and covers, but for times sake I'll have Ivan Reis do interiors just because he's my favorite artist DC has right now and he does great work on Green Lantern.