I think the Ho-Oh/Lugia theory revolving around Megas would be really interesting. The two are clearly tied into sun and moon lore and, given GF's new tendency to re-vamp older ideas I don't see it being completely impossible. I think the only problem comes in the marketing of the games, a lot of people want to see brand-new Pokemon on front-covers... then again... if they completely nail mega-evolutions for the pair, what better way to get old and new fans interested?
Personally, I think they'll go for 2 new legendaries
Sun: fire/psychic or fairy
Moon: water/???
I'm hoping for a new eeveelution, obviously the logical choice for this is dragon type, be interesting to see how they make this work given the very feline appearance but I'm sure they can find a way
I myself still like the Fire/Electric typing, but Fire/Psychic or Fire/Fairy, as listed above, are also good contenders in terms of mythology inspirations. I also like the Fire/Grass option, which was never used and goes great with whatever the legendary mascot of Pokémon Moon is, which is hopefully Water-type at the very least. If its Fire/Fairy, then the only two typings that could work, would be Water/Dark providing a perfect counterbalance, or a one sided advantage with Water/Steel.