My tone may be flippant but I do actually have to know: what has Bolas got beyond 'God damn I'm evil'?
I can only speak for myself, of course, but there are several things I like about Nicol Bolas. Firstly, he is an unbelievably ancient being, one who has seen and done more than all of the neo-walkers put together, hundreds of times over. I am always interested in the nature of the minds of characters that ancient, because we have no perspective on what a life that long is like.
We view Bolas as evil because others' lives mean nothing to him beyond their usefulness to his plans. And while I would never justify that sort of casual disregard for life, I would pose the question of 'can we really blame him?' First of all, he is a dragon, and dragons are apex predators in almost any Magic setting they are found in. But even if he were not, his lifespan alone must twist the perspective on the value of life. The current average life expectancy for humans on Earth is between 70 and 80 years, give or take depending on where and how you live. Imagine if you live to 100. Most of the people you know will have died before you, and you will have watched them go. Now, imagine you live 200 years, and go through it two or three times more. Now double it again, and again. People keep dying, and instead of living 70% of your lifespan, they're living less than 10%, and that's only at 800 years, not the literal thousands and tens of thousands we are talking about with Bolas. It must be difficult if not impossible to develop any sort of relationship with - or empathy for - lives that you know full well will end in what is, compared to your lifespan, a blink of an eye.
Bolas also fits into the schemer trope, which I have always liked, and is my favorite color combination (well, he's my favorite color combination AND red, I guess). He's also an Elder Dragon and, though we've been given very little information about it, the Elder Dragon War was one of the first pieces of MTG flavor that drew me in.
And finally, the other thing I really like about Bolas is that he's beating the Gatewatch. Now, I know that sounds like "oh, he hates the Gatewatch so anything that beats them is good," but hear me out. The Gatewatch has been built around the concept of comic superhero teams like the X-Men and the Avengers, because of their popularity. My belief in terms of comic book stories, and even fantasy writing in general, is heavily tied to the Hero's Journey. I dislike heroes that are stronger than their opponents, because there is either no tension whatsoever, or the tension comes from convenient external sources like kidnapped hostages or loved ones. When I read a Superman comic, particularly an early one, I feel little to no tension that Superman would in any way lose. He's too strong, and too much stronger than his foes. When I read an X-Men comic, even the early ones, there's a good chance someone's going to lose. Maybe it won't be the team as a whole, but individually, someone's likely to get beaten. I like that tension of not knowing what's going to happen before I read it.
And then, every so often, the team runs into a Magneto, or Apocalypse, and gets curb stomped, and then there's some REAL tension as to what will happen.
The Gatewatch, thus far, have had it easy. They face three ancient, unfathomably powerful beings on Zendikar. One just leaves, and the Gatewatch tackle the other two with relative ease and little to no personal struggle or loss. The only difficult part came from Ob, who had virtually nothing to do with the overarching story in any real sense. On Innistrad, their problems literally solved themselves. On Kaladesh, they waltzed through a mostly pointless revolution. I think it's fair to say that I have felt no sense of tension whatsoever from almost any Gatewatch story I have ever read. There is no sense of danger to them.
Nicol Bolas provides that danger, and not just for the Gatewatch, but for the Multiverse as a whole. Do you have a favorite plane floating out there? If you do, now Bolas showing up on that plane is a legitimate threat. Like Magneto was to the X-Men, like Doomsday for Superman, Bolas has been established as a serious threat. In some ways, Bolas is one of the few reasons for the Gatewatch to even exist. There's no point in watching a gate if nothing can get through anyway. Bolas can shatter the gate and everything on the other side.