Personally, I hate Hotm, they take space and if you pull a lot you can have too much. There a way better heroes and it's not worth it to level them. That being said Mystia is my favorite bcs useful against Bearnadette but even then I never brought her to level 90, missing AA, and it's a risk to use her. Although if she can fire first what a relief!
For sure they should keep costume for Hotm. I always wanted Perseus, and I finally got him, not sure how long it will take to level though. I also Maxed May but it would have been better to keep her at level 85. It was my first growth hero but now I have much better. It wouldn't have hurt the game to make her better.
With the dupe thing it's tempting to level just to level fast but then you're stuck at level 70 bcs don't waste your ascension mats on that. I get that for other rosters it's not the same, and SG thinks of all it's players. But if anything they should make it fully leveled. This way it could be really exciting for most players giving it a reason why it's 
And for others it would be a leveling boost and 10 gems for the museum without wasting your precious mats. SG would not lose profit (for spenders it's a small by-product) and probably keep more players playing. Win/Win IMO
