The issue kind of comes down to 'are we going to differ a thematic repeat enough for it to not be an upgrade?'. Black Frost Anivia, for instance, is where the corruption of the Freljord radically impacts her. So, it's an ice theme, but it's a different version that's not quite an upgrade over the base. Or, in other words, base 'Good' Anivia and then 'Evil' Anivia. Whether or not the extreme personality difference, and the resulting change, qualifies as a significant difference or not is ... up for subjective debate.
On the logical other end of the spectrum, another fire skin for Brand would be hard pressed to be very 'different'. You could, at most, have him become his original spirit of fire, but that's just an upgrade. There's no difference between imprisoned Brand or released Brand. Were you to do that route, a Gangplank style story evolution would be more suitable than a skin, per say.
Diana, in the middle of these two, is kind of funny. She's the avatar of the moon, but there are many different ways to represent her. Lunar Revel, Blood Moon, etcetra, all serve to portray the same Diana in different views. It's essentially clothes swapping at that point, where the implementation of style and the 'is this worth having a skin for?' become more paramount. You'd have to go a step further in altering Diana's personality to really avoid the clothes swap problem, but it's an uphill struggle.
Some people like costume swaps, some others want total do overs ... market research will show where the main incentive to develop crops up from, really. There's arguments in favor of either camp.