I honestly think a lot of people here have a point and it can't be just a cameo. It can't be pointless. Marvel's learned from their mistakes of the past, so even though some of the things in phase 2 may be jam packed, I'm thinking they'll be integrated a little better. That's what Feige could be talking about by saying IM3 is a nice mixture of things. Like what we're seeing with AIM, Mandarin, Firepower, Coldblood, Iron Patriot-sort of, Extremis, Rescue, Radioactive Man, all appearing in one movie.
I've attempted to work in Jan because I've come across sources I personally know, and that are involved in some small way- telling me she's Jan. I still have no real clue how that would work.
I'm just thinking of ways that they could be worked in so that it's not as pointless as Widow or Hawkeye. If they were already working for AIM, instead of are it makes it easier. Sort of like they're casually mentioned by both AIM and Shield in the TV show because they've worked for each organization in the past.
The show could detail the history of AIM and Shield every once in a while, but only make reference to people like Pym and Janet who were a big part of the 80's. Pym is knowwhere to be seen until after the GoTG.
My full theory, for those who want to read it (in spoiler tags so it doesn't take up room):
Spoiler!!! Click to Read!:
In my full theory her presence won't be completely explained until the Ant Man movie. You could see a little of AIM's background with them in the TV show, but mainly Shield's past with AIM. Hank and Jan won't be seen together until Avengers 2. When they're reunited after being separated by dimensions as well as space and time (due to Ultron's plot to stop their existence in this timeline through what they were working on for AIM). Janet and Pym were working on a top-secret project for AIM after leaving Shield. They unlocked Pym particles and almost unlocked sub-space time-travel through neutrinos. AIM is trying to retrieve Red Skull from the past--they're secretly a modern tool for Hydra (but we don't know this for sure or find this out until Avengers 2).
Shortly after making himself big again, Pym and Janet attempt to flee from AIM/Ultron prototype 2 to the future with the device they were being forced to build for AIM. As well as the Pym particle research (which he destroys and is now only within his mind). Jan decides to biologically enhance herself with the Pym particles even though Pym hasn't done it to himself. There's not enough time to build her a suit, and they're arguing because Ultron is taking over AIM... She injects herself with nano-biological implants Pym was working on for himself. They allow her to size change at will with the Pym particles, without a suit. Pym doesn't have the biological enhancements that Jan has at the end of the story... This leads to Jan ending up where they intended, and Pym somewhere else. They make themselves the size of neutrinos, get the time machine functioning, and take Ultron out of the past (which has repercussions for the future of the series and would be how Ultron "leads" the masters in Avengers 2). This was Shield's not so recent mistake of letting that technology fall into AIM's hands. In the Avengers Fury says "Shield made a lot of mistakes, some of them recent".
Janet winds up at some point in Advanced Idea Mechanic's future before the time of IM3 and infiltrates them to search for Pym. Stark knows nothing about AIM's past, and it becomes clear to her at some point that they didn't finish the time-travel research. Mandarin like to act like an Oracle but he knows nothing of Shield and AIM's true past, only what he predicts about the future based on past Empires. She quickly gets wrapped up with Stark and Shield. Pym ended up somewhere much further away because he hasn't altered himself biologically yet. He slipped between time and dimensions as they were traveling... The Yellowjacket suit he's been building to time-travel in malfunctions. Sending Jan to the present as they travel & thrusting him into the GoTG movie for a cameo as Yellowjacket before we're fully introduced to how this Pym has been traveling time and space. He went into the negative spaces between time and dimensions because he wasn't able to directly hitch a ride on neutrinos like Jan was. Pym from the 80's has been missing for a long time, never heard from again, but returns after Lang dies in the present. With a way to send villains into the negative zone.
The first time people would see Pym they wouldn't know it was him, unless they're familiar with Yellowjacket. Jan becomes a big part of the after credits but doesn't know this is where he ended up. We find out a little about her search for him in CA:Winter Soldier. Yellowjacket's raygun is tuned to the negative zone and makes a brief cameo at Knowwhere in GoTG with the future Guardians. They contact one of the 21st century Guardians about Thanos. Adding more mystery to the Pym/Janet separation until it's explained through Avengers 2 and the Ant Man movie. Allowing Pym's Yellowjacket persona to be briefly expressed. When we see him in Avengers 2 he's becoming Ant Man/Giant Man as a hero for the first time, and Lang's powers introduce him to the story. Lang would be included from the beginning of Avengers 2 but dies. Thankfully a force from beyond time has sent Pym back to the 21st century where he's needed to protect the galaxy and defeat Ultron/Thanos.
I do agree that if she's Janet for no other purpose than as a cameo to set her up, then it would suck.