Solas's big fear is that the elves are slowly losing themselves and their culture. Even the Dalish, who are actually trying to preserve their heritage, have gotten so many things wrong. Sera's the end product of this decline. The fact that she doesn't even want to entertain the idea that what she's feeling could be an "elf thing" speaks to this. In previous banters, Solas tries to talk her through these feelings, which are a result of an elf's inherent connection to the Fade, but she either ignores him or is hostile towards the idea. So the line "you are the furthest from what you were meant to be" is his frustration at her unwillingness to listen finally coming to a head. She isn't 'serving her purpose' to the extent that she is actively trying to cut herself off from her heritage. And Sera just checks out because she's heard it all before.
If you put it in the context of all of the banters they have together, it's pretty clear that he isn't talking about any sort of mystical god-being that she may or may not be harboring inside of her.
Well, Sera doesn't fear 'nothing' so much as the prospect of nothingness. She suffers from some pretty major existential dread, which is why she holds on so tightly to the idea of Andraste and the Maker, even though she's not the hugest fan of the Chantry. That's also why she's so insistent on living in the moment. She doesn't want to think too hard because that leads to thinking about things that she's too scared to face.
Really, the beauty of Sera as a character is that she isn't really anybody special. She's a person who's been caught up in these world-changing events and decided to do something about it rather than just sitting around. I mean, you don't even have to recruit her. You can get rid of her whenever you want. She isn't essential to the story in any way. But she's a compelling character precisely because she's not special. She owns that, and uses it as a strength.
Actually, Solas' biggest fear is dying alone, but that's not important.
I think we've basically come to an impasse as to what Solas actually means. I read the conversations one way, you the other, and our interpretations of Solas are different. Let's just agree to disagree.
However, I would argue that a lot of characters are not "essential" to the story in any way; Vivi, Cole, The Iron Bull, and Blackwall are all companions that you don't need hanging around, nor do they ever do anything significantly important. If you want to key out "Isn't special", The Iron Bull is your man. And considering that I don't have to have anybody except Varric, Solas, and Cassandra, nobody gets bonus points for being disposable or indispensable.
And, of course, what I like about Sera and what you like about Sera aren't going to be the same thing.
However, I do think you're wrong that Sera doesn't want to think too hard; a lot of her stuff/beliefs hit pretty hard up there in terms of philosophy and thought, so she's obviously thought a lot about it. She's also not actually around for fun or anything; you can convince her to stay for the people when she threatens to leave, and she's like "******, I thought I was here for fun, but I guess I wasn't all along. ****." She's also done a number of really suspicious things, like somehow defacing your codex of her everytime something's been added (how does she KNOW?) and isn't forthright about her past even when probed. I think that really, what she doesn't want to think too hard on is herself; everything and everybody else is fair game.
And of course Sera is special; picked up a bow with little to no training, can keep up a fight with Cassandra of all people (warrior for years, trained under the best, constantly at it...then this chick shows up with arrows and can slay a motherfucking dragon at level 23 with her focus on nightmare. WTF.) Sera is also a member of Red Jenny, a group that we knew next to nothing about in until she said something. Strangely perceptive, has a tavern song dedicated to her, was adopted by a noble despite being an elf. In terms of personality, I'd say that she's as unique as they come by.
Sera might say she is normal people, but everything about her doesn't say, "normal people."