As a scifi aficionado, I would enjoy seeing Erica and Kai together.
However, I understand the writers wanting Erica to remain grounded as an ordinary woman that the audience can relate to. I am pretty sure many young women look towards Erica as a role model. Giving Erica a boyfriend from the future may be pushing the non-scifi audience's suspension of belief, and risks turning Erica into some kind of super heroine. In fact, this is part of the reason why I suspect Erica will ultimately abandon being a Doctor in favor of putting all her efforts into her family and 50/50.
I also suspect we won't know what that future disaster is. However, I do hope the producers do answer why Doctor Nadiaa keeps trying to interfere in Erica's therapy, and how Doctor Arthur fits into the whole scheme of things.
See, here's the thing: a story's direction should never be determined by the artist's perception of what the assumed demographic may want. What, women supposedly don't like sci-fi? Then don't write a sci-fi show for them! A story has to go where a story wants to go, and clearly Being Erica's most fascinating, most compelling, and most unexplored plotlines at the moment are precisely those sci-fi questions that the show itself did indeed bring up. Everything else now is done. Erica is a healthy human being. She has dealt with her regrets. She has gained confidence in her abilities. That plotline is finished. It just is. They finished it.
Actually, I was more thinking along the lines of Doctor Who, where The Doctor always has a human companion who travels with him but ultimately returns to live the rest of their lives relatively normal life back on Earth. The Doctor is alien and can do all sorts of wierd stuff, but it is through the human companion that the audience relates and empathizes with.
In Being Erica, Erica is the "human" aspect to all the time travel strangeness. Her role in the show is to simply improve her current life, not to do strange things like permanently change the time line or have a boyfriend from the future.
Now, the macrocosm is left. There's a disaster coming up. Do I believe that Erica could just forget she knows she may be dead in 10 years? Not in this universe. No way. It should be foremost on her mind. Also foremost on her mind should be the question: why me? Why anyone? Why are the Doctors doing this? What's the catch? What's in it for them? She would no longer be satisfied with evasive answers. The show has asked compelling questions. It has one season to answer them.
When Kai first revealed this future disaster and the possibility that Erica dies in 10 years, I was thinking "how is this different from her mother suffering cancer?". Lots of terminal cancer patients have a limited period of time left to live. These cancer patients can go asking why me? why anyone? why is god doing this? but ultimately this is a futile line of questioning that give no answers. Ultimately most choose to live their lives to the fullest and/or settle their affairs. I believe this would be Erica's response too. Live life to the fullest, fulfill any incomplete dreams and spend whatever time she has left with loved ones.
Erica won't be tearing through time and space seeking to "find out why". She just isn't that kind of person and this isn't that kind of show.
Maybe it's just me, but the second I heard Kai say that he couldn't find her in 2019 I thought to myself...
"She's probably a Doctor now and living outside of time in the conventional sense."
The other thing I keep coming back to is... Dr. Tom jumped off that ledge. As far as I can imagine it, the Doctors are not dead per se, but no longer a part of the mortal world.
This is somewhat spoiler-ish, but Doctors including Doctor Tom are alive and part of the mortal world.