Wow, it's really cool to see that Leona concept still getting support two years after I drew it - back when I was just a LoL fan with big dreams. Obviously Leona, Diana and their relationship are a very popular part of the League and that's why it keeps resurfacing! I've been looking for an excuse to contribute to the forums since joining Riot proper and this seems like a great place to start >:D
While Diana and Leona are tragically opposed, the idea of an 'Eclipse' that involves a balance of the Moon and the Sun is a pretty interesting idea and a very strong thematic device. I originally drew that concept as a response to IronStylus asking the forums what we thought an Eclipse Leona would look like and, having a tendency to make things dark and evil (Yes, RiotPenguin and I have formed an alliance with the sole purpose of corrupting every LoL champ), I leaned towards a sinister skin inspired by a solar eclipse.
I'm not so sure that the same design language would work for Diana, although perhaps a version that really plays up the solar fire and the warm accents would work well as a counterbalance to Leona. I also see time and time again the polarising effect the covered face has on players and wonder if it's because Leona is just too beautiful or if my execution, though just a rough sketch, was too crude. Looking at it now does make me cringe a little and Nitenz's Diana is a much nicer design.
The reverse skins by Vegacolors are really well executed. I love how you can still identify their faces despite them having radically different hair colour/armour. I don't think we could ever do designs exactly like this due to issues with how they would read in-game. One important factor we have to always think about is "If this champion pops out of a bush do I immediately recognise who it is?" and while it's one thing to make a departure from the base skin, it's another to look just like another existing champion.
It is truly a wonderful thought to imagine them side by side though, even in an alternate reality. With that in mind do you have any thoughts on how else we could portray them, fighting as one?