Who is Nemesis and why would she possibly have the Time gem? Well ladies and gentlmen, meet the creator of the infinity stones. Not just the creator, but the stones are literally her. It is believed that Nemesis is older than the Marvel universe. She killed herself and transformed into the infinity stones/gems. This character is a slight reach, especially since there hasn't been any talk about the character. However it would fit the current model for naming the stones. And she literally is the stones, so dealing with Infinity War it could happen.
If you haven't heard the new theory, lets look at the graphic below to explain:
So apparently the name of the what holds the stones/gems are spelling out the name Thanos, our future big baddy. Is it possible Nemesis exists in the MCU and wants to put herself together again? This would actually be a really cool concept. A third party scenario of sorts that is after the stones. Maybe Nemesis has utilized the Time stone to manifest a body of sorts. With each stone she would grow more powerful. A three way battle between Avengers, Thanos and Nemesis.
Here is a little more info on Nemesis from ComicVine
Nemesis has an origin shrouded in mystery. It is said that she was the the sentient being that created the Infinity Gems, it is also said that she might be older than the current universe. At a moment in her history she committed a form of suicide and transformed herself into six very powerful gems, the Infinity Gems. Each of the gems possessed an incredible and different power, which the wielder of that gem would acquire.
This is all just a fan theory so please take it with a grain of salt. I was just researching what the "N" could stand for and came across this interesting tidbit. What do you think of this theory?