Sorry for the delayed response. I've been getting over a bout of the flu and every time I tried to respond previously, I lost my train of thought. Hopefully I'll have better luck today.
I do think Elena, besides being the lynchpin of the triangle, has been getting outside arcs.
She has had many arcs but I wouldn't say that any of them were truly outside of the triangle. Even her turning her humanity off was primarily about the two main plot points of all Elena centric stories—her inability to deal with loss and so the guys and her friends have to protect her from it; and her choice in the triangle. In this particular case, the turning off of her humanity was probably her strongest arc because it did lead to some actual character development in that she made the decision to remain a vampire and be with Damon. Of course she still didn't completely let go of Stefan so it has never been a true choice.
Elena turning her humanity off made the brothers even more determined to find the vampire cure and restore her humanity which in the end let to Silas and so far the Silas storyline has proven to be about one person and one person only and that is Stefan.
There wasn`t as much background info on her early on than Stefan, Damon and Katherine but in fairness to Elena, they are each more than a hundred years old so there is lots more to draw from than a sixteen-year-old. And of course they were the mysterious new vamps who had to be established to the audience via their flashback story in Season 1 and even a bit of 2.
This is a valid point —especially the part about the introduction of the new vampire characters eating up the screen time. However, I still say that even with the many new vampires that were introduced, the brothers along with Katherine, Jeremy, Caroline and even Tyler have had more character exposition and development than Elena. The latter two are the same age so IMO, her being 16 does not really hold as an explanation.
I think the main problem is that being the center of the triangle has just eaten the character of Elena alive as usually happens in such instances. I have yet to see the character at the center of a long term TV triangle not be seriously damage as the writers twist the character into knots to keep the triangle going. Since I see signs of the writers pulling together obvious and not so obvious plot threads that they threw out there out since the first season, I suspect that the triangle and the show will probably be coming to an end soon.
Caroline fares horribly plot-wise, even worse than Bonnie and Jeremy who at least had actual, if not really well-written or concluded arcs of their own, being consumed by magic/expression, seeing ghosts, hunterdom etc. Not sure Caroline ever had one. Her transition into a vampire during Season 2 was quite fun but other than that she seems to be Lexi 2.0 for Stefan, the resident Stelena cheerleader, girlfriend of Tyler of the revolving door powers and object of Klaus`affection. I think only Matt has had it worse.
I actually forgot about Jeremy because the actor other than being good looking with a hot bod has not made any impression on me. However, you are correct in that he has had strong character arcs. Actually, when I think about it, he has probably had some of the strongest arcs of anyone in the series. It was more interesting the first three season when he was dealing with all the usual teenage angst like fitting in, first love, and his attempted suicide.
Bonnie's story on the other hand has been a lot of plot points whose main purpose is to help or save the core three and while I would argue that they were not true character arcs it was not for lack of potential. It seems to me that the potential for really strong stories were there for Bonnie but the writers have had no interest in writing for her and so every time the story called for true development on screen for the character, they sent her off to visit a relative and so did not have to write the story. They then had her show up at the end to save the day and set up the next plot point. Even the current plot point of her being the anchor is rife with story potential. I can think of several off the top of my head but I would bet anything that it won't really lead to anything.
I think that we will have to agree to disagree on Caroline because I've seen true character arcs and development for her since the first season in her relationship with her mom, father, Tyler, Stefan and even Klaus.
I actually think the only person who had an actual arc - even if that arc was mostly "become a better person for Elena" - is Damon. Apparently the writers liked that arc so much they're now doing it again!
Damon had strong arc in Season 1 & 2 but became stagnant in 3 & 4. The writers could be recycling stories with this Enzo arc but I hope not and it leads to something else down the road. I'm sure that in the short term you are right and it will be about him getting back together with Elena but I really hope that I'm also right and in the long term she ends up with Matt and not with either of the brothers because to me that is the only satisfying conclusion to the series.