I feel Luxray should have been made partially a dark-type.
Whenever we see this Pokémon, I believe many of us would agree that it reminds us of this particular type-group. Due to design reasons, I feel it would've made sense. Aside from that, it's also got access to a ton of dark attacks in its arsenal, and if they had turned Luxray part dark, they would have gained some damage boost, making it more viable on competitive side of things. In a way, it would've been a win-win!
I'm afraid I have to disagree with this one. I've seen a lot of people argue this, especially for an Electric/Dark mega, but it doesn't make any sense if you look at the lore of Luxray. Just because a Pokemon looks a little scary and has a lot of black in its color scheme, doesn't mean it should be a Dark-type.
Luxray is a Pokemon known for x-rays, and illumination. It has glowing, gleaming eyes, and has a very light-focused theme. Its star-shaped tail especially gives that impression. Something so light-oriented shouldn't really have a place as a Dark-type. Also, it doesn't really seem to have a particularly sneaky and dark personality emphasized. Gyarados' Mega got the Dark-typing because of how absolutely atrocious and destructive its personality was.
Also, Pokemon can have some minor traits of another type without having to be that type. Like Ninetales and Golduck. I think it makes things more interesting and unexpected that way. Just because something seems one way, doesn't mean it is. Not every Pokemon has to be a dual-type.
I think most typings make sense in their own weird way, to be honest. Although there is the issue of having types based on actual creatures. (Dragons, Bugs, Fairies) And that leaves an issue of conflict if you can only be one type. But Flying is also a good replacement for Dragon, to be honest, as most Dragons fly. Perhaps those things are more oriented towards the skies than towards dragon-element energy.
The only one that catches me off-guard is Flabebe though. That should definitely be a Fairy/Grass type. It has more Grass-type traits than a non-Grass-type should, learning so many Grass-type moves on level-up, and its lore emphasizing the importance of its flower.