Tirek wants more magical power and to destroy everything while Cozy Glow and Chrysalis want to conquer Equestria. As for Nightmare Moon, she had an interesting motivation, but that doesn't mean she was a good villain. Princess Luna felt unloved because ponies preferred to sleep during the night. Since that was all we were given, and since ponies are generally diurnal, even that motivation is questionable without expansion. And NIghtmare Moon wasn't exactly an impressive threat during the series premiere. In terms of threat level, the villain trio did well in the episode since they had plenty of magical power and proved to be threats. However, that didn't work out as well as it should have because of a persisting problem the show has when trying to present the ponies dealing with the threat.
That's a fair point. Unity and respect isn't just about having common interests; it's also about being able to accept and even work with differences. And I suppose you're saying that there isn't enough to distinguish one creature race from another other than the others presumably being less familiar with friendship? I do find the "less familiar with friendship" stuff strange as it's a universal concept. Or at least getting help from others who aren't like yourself would be universal.
The former two would never happen because the writers don't know what to do with the sisters in terms of their position as rulers. I don't think they would need to depict a real life example of a political job, but the problem the show has is that I don't even know what they're good for. Luna does have her dream powers, but she doesn't need that to be a ruler.
Yes, I'll cover "The Last Problem". I don't know when I will get to it, but I will at some point.