Epic has three main stories- Epic Adventure, Bored and The (Epic) Tales of Bob Awesomness (though as Epic herself I can personally tell you there will be at least one more). Each story has its own characters and stuff. She also has her fanfics on Wattpad. There are plenty of things linking her current three stories, listed below.
The taco shirt, ah yes. The taco shirt is a white T-shirt that shows up a lot in the stories, especially Epic Adventure (what with Dark Epic, Light Epic, Darkipic and Pinepic all having their own versions). The three direct protagonists of the stories- Bob, Epic and Jenna- each have their own shirt that is an exact copy (exception of the sleeve colors, Epic being red, Bob being blue and Jenna being green). There is one more character that's been revealed recently. His name and story involvment is unknown, but he has his own taco shirt with yellow sleeves. He seems to be a ghost in every drawing with a strange gray/black aura constantly surrounding him and being drawn as floating or flying.
The beanie that Epic wears (also known as Mr. Hat or Bean) shows up often throughout the stories (again with all the variations Epic's alternate halves have). It is mentioned in Bored at one point and is directly talked about in TEToBA. It does talk to Epic, though the other beanies don't talk to their owners. The beanie is the logo of Broken Square Productions.
There's plenty of pineapples in TEToBA and Epic Adventure, but not so much in Bored. In Epic Adventure two characters- Darkipic (Gavin) and Pinepic- both can become floating, talking pineapples. TEToBA typically mentions pineapples every once in a while- in the Pineapple Castle (which hasn't been mentioned yet, spoilers, sorry). In Bored, on the cover there is a pineapple floating in space. Also, the book that Pete's constantly reading is actually TEToBA.
Most of the bad guys in the stories often need to invest in better security measures . People such as Kone, Riley and the Shadows let the good guys slip right out of their hands, to the annoyance of themselves and their superiors (if they answer to any). Their security measures that they need are often the best security measures around, and will never fail. Even if they were to fail they could just add even more advanced security measures that are undoubtedly going to work. Even if one got past these advanced [measures] they would have to the the most hardboiled escapee ever.
The first security measures are considered a reference to Andrew Hussie's Problem Sleuth.
The stories are broken into Acts, but each story is only one Act. This is yet another link between the three stories, having Epic Adventure/TEToBA/Bored be Acts 1, 2 and 3, respectively. It is not known if all of this will add up or not at one point.