With Avengers: Infinity War parts 1&2 coming up in 2018 and 2019 i figured that i would offer my thoughts on who can be important in the storyline of these highly anticipated Marvel movies (in no particular order)
Where else to start then the leader of the Guardians Of The Galaxy? As we saw in the 2014 movie Peter Jason Quill was abducted from Earth by Yondu and his team of Ravagers. Who then 26 years later (spoiler alert) finds the Orb which houses the Power gem of the infinity stones. How he can be useful is that at the end of GOTG we saw him able to hold the power stone for a small amount of time without being immediately dissolved into nothingness. Closer to the end we find out that his father (who will not be J'Son of Spartax [confirmed by director James Gunn]) is something ancient that has never been seen on Xandar. But you would think if it was J'Son of the Spartoi empire that the head of the Nova corps would have heard of it (considering that the Nova corps is pretty much like a galactic police force).
2. Capt. America
The leader of the Avengers team. Capt. America was the first Avenger to stumble upon a infinity stone (which if you need reminding is The Tesseract, which is The Space Stone) As the plot was focused around getting the Tesseract back from the hands of HYDRA and their leader Red Skull. We last see the Tesseract getting picked up by a drone of Howard Stark but after shooting the Red Skull off to someplace unkown... (spoiler alert... Maybe the same place where Jemma Simmons was seen at the end of S3E1 of Agents Of SHIELD) Then Capt. America was revived in the modern time after being frozen for 70+ years.
3. The Vision
The Vision who was first introduced in this years Avengers: Age Of Ultron. But who was first (spoiler alert) J.A.R.V.I.S Tony Stark's AI system. Who if you remember (again spoiler alert) had the Mind gem from Loki's scepter embedded in his forehead which gave him life which was also brought to life by the help of Thor. Also The Vision who is also the only one so far of The Avengers group (In The MCU) worthy to wield Thor's hammer. As Thor Stated near the end of AoU
Thor: He can wield the Hammer, he can keep the Mind stone.
Or something along the lines of that. Now if you know The Infinity Gauntlet needs all of the Stones to work as Thanos wants it to work. Does that mean Vision will die? or will the fan theory that Vision is the MCU's version on Adam Warlock will turn evil from the last Ultron drone he destroyed/Absorbed and give the mind stone over to Thanos?
4. Dr. Strange
Yes Dr. Stephan Strange the Sorcerer Supreme of Earth. Even though his movie will not come out until Nov of 2016 i have a theory that either the soul, or the time gem (the last two gems out of the six) will show up in his origin story movie. Being as though since he is the Sorcerer Supreme of Earth who can travel to different Dimensions . Now who knows? Both might show up, or neither can show up, we have no idea what Marvel has in store for us! But with that being said Dr. Strange could be of importance since he dwells in the land of magic (which we have seen a taste of with the Scarlet Witch from AoU) he could be of great importance with helping the Avengers defeat Thanos.
Some of the muscle of the Guardians Of The Galaxy. In the comics Drax and Thanos has gone face to face multiple times. Sometimes even killing each other or only one being victorious. Drax's sole purpose which he said near the end of Guardians Of The Galaxy was to destroy Thanos to avenge his Dead wife and daughter. Which he somewhat did when he helped destroyed Ronan The Accuser with the power stone. Drax has a grudge against Thanos for deploying Ronan to his home world and killing his family.
The adoptive daughter of Thanos. Gamora is, or was Thanos' favorite daughter over Nebula. Gamora possibly again i'll say possibly knows all of Thanos' weaknesses or plans to get all the infinity stones. Gamora being some of the brains of the GOTG and some of the muscle could possibly go toe to toe with Thanos (without the Gauntlet) but being his adoptive daughter for sometime she might know how he operates and again, what some of his weaknesses are.
The mighty Avenger, Thor will be invaluable to The Avengers for his cosmic powers and knowing what the infinity stones are and possibly what they can do. Thor having come into contact with the Aether (now confirmed as the Reality stone) has now come into contact with 3 of the 4 infinity stones that have come into play (and possibly a fourth if the time or soul gem shows up in his third movie). Now Thor knows the Power stone has reveled itself thanks to the help of the Norns which is in the deleted scenes of the AoU blu-ray/DVD release. Thor knows 4 of the 6 infinity stones are in play and that some higher force will be coming to collect them.
We will see how all this plays out when Phase 3 of the Marvel Cinematic Universe starts on May 6th 2016 with Capt. America Civil War. Thank you for reading my first post and please comment if you think i've missed something or a character that you might think will be of great importance in the two part Avengers Infinity War!