I would love it if they did that. They spent so much time setting up the good guys, why not the badguy. So when avengers 3 premiers everyone who saw thanos quest will just nod and say " avengers are fucked..." And you have no hope for them winning. Make avengers 3 a 2 part movie half a year apart.
So let's imagine they actually did make thanos quest a stand alone movie, you would be able to introduce thanos, death, the gems and it's power. And focus on the tactical prowess and utter lunacy of thanos. Make him be intriguing and also respect his enormous level of power, and what he is about to obtain. Loki gives him the gauntlet, and feeds him the info about the gems.
Thanos quest is done, and now you want to set up the avengers 3. Like i said a 2 parter, because you cannot do infinity gauntlet justice in 1 movie.
Open with thanos angry as fuck at death for shunning him after he sort of betrayed her, her envoy sais something like "you betrayed your mistress, you revere her, you love her, but you went past her and became more powerful that death itself, Loki (replacing mephisto) whispers something in thanos' ear wich angers thanos and he sends a power surge through the galaxy that litterrally knocks silver surfer off his board, and surfer immediately realises what happened and he sets off to earth where he finds the guardians of the galaxy ship adrift after being hit by the same ripple. He boards the ship and sees the guardians scramble trying to patch up the ship in panic yelling they have to set course to earth to inform the avengers and especially thor what happened, and the danger that is ahead. Surfer goed ahead and crash lands in the sanctum sanctorum of dr. Strange, the guardians land on earth almost litterrally crashing into a helicarrier. They get intercepted by nick fury and they explain who they are and what is about to happen.
The avengers get assembled, add some appearances by captain marvel, black panther and lets just fantasize the fantastic four are no longer property of fox, they get called up too. Add nova? Quasar? You need some space guys i guess.
Dr. Strange appears in a big flash with a severely exhausted silver surfer in the middle of them all and everything gets explained about who thanos is and what terrifying weapon he wields. Thor recognises the gauntlet they speak of and ports away trying to find loki, who isn't there, he informs odin, who assembles the other asgardians to prepare an attack.
Stark and reed richards repair the guardians ship and they all set off into space, maybe add ronan the accuser who apparently is being introduced in the guardians of the galaxy movie, and gladiator and some other notable alien heroes( no beta ray bill!) to make the force that is off to attack thanos more impressive.
And the battle starts, with dr. Strange keeping everyone alive in space. Thanos just laughs and shows some power, where loki gives the heroes a fighting chance with his manipulation where he informs thanos that death might be impressed if he gives the heroes a fighting chance, thanos drunk with power agrees and tells loki he will only use a fraction of the power he has.
A lot of fighting goes on, and thanos almost loses the gauntlet to silver surfer who flies in hyper speed and narrowly misses the gauntlet. Thanos becomes enraged and uses his full potential to defeat the heroes. End the first part with ships broken and adrift, heroes heavily wounded and a total devastation.....credits.
Part 2 begins in a white space, odin and a heavily beaten thor make their plea to a voice, who answers they will not interfere and risk destroying all reality. Eternity will not join the battle(wich solves that problem) but they will send someone to aid in battle. Big flash of light, and we're being dropped in the middle of the guardians spaceship that made their escape with a handful of avengers and a lot of heavily wounded heroes. A deafening voice scares the everliving crap out of silver surfer, who explains to dr strange that his master has entered the battle. Queue a big round orb appearing, and galactus stepping outside.
Galactus was followed by 2 celestials, who are explained by silver surfer to be caretakers of the universe, a race of gigantic entities. The celestials move towards thanos, thanos floating in space with energy dripping off his hands in a pose that screams "bring it on!" His eyes on fire he flies headfirst into one of the celestials and like a bullet enters and exits a celestial chest. The other does a thunderclap swatting thanos like a fly. His hands explode and thanos is now even more angry. Galactus hurls a few planets at thanos, and sends a meteor shower at him. Thanos flies to galactus and odin and the aesir appear, with thor and a person that also gets introduced in guardians of the galaxy, thanos' daughter nebula. She moves towards "dad" and in an unguarded moment thanos loses the gauntlet to nebula. She gets overpowered by the gems and inexperienced as she is wielding it, the time for a last ditch effort to defeat the wielder of the gauntlet succeeds as mjolnir clocks her in the side of her head, all the remaining heroes give everything they have left in bringing her down, and loki, who turned coat as he sees thanos getting defeated, takes the gauntlet. Thor and loki stand face to face and odin sides thor and destroys the gauntlet.
Drax the destroyer sees his chance to kill thanos and plunges an aesir spear in thanos' chest. Death walks up to thanos and sends him off to his death, and whisk everyone away to their respective places in the galaxy.
That is how i would want to see it done. Despite the few property problems, this way you can leave out a lot of cannonfodder superheroes, nix the xmen, nix spiderman who has no business there, and you don't have to bothe with adam warlock and the infinity watch. Adam warlock is hell to introduce, as it's such a complicated character....
So this was my fantasy screenplay lol.