Boss size... i like big bosses, Shadow of the Colossus still fills me with an intense feeling when i run towards the biggest and baddest of them like an insignificant human flea, but again like TP and WW showed, bigger doesn't mean better.
Attacks, I want bosses to come at me and i can't believe I'm saying this but Scervo form SS did what i wanted. Kind of. In SS if you missed Scervo would hound you and make you wish you hadn't, for me Scervo was the hardest part of SS because he was so aggressive and i'd like more bosses to be that aggressive in the future, not so much his battle set up but the way he didn't give you much of a chance or opening.
So yeah.. make it feel epic and make it feel like the boss actually wants you dead.
Imagine a beast-style Ganon (ala Ocarina of Time) about the size of Barba (the sixth Colossus, beareded minotaur) or even the size of Malus (the final boss, the great FRIGGIN' TOWER)!
Parody Oekaki art I did a while back:
The problem here, if he was like any of the Colossi, would be speed. Ganon could possibly make up for it with magic somehow, or just be made a quick giant.
Scervo wasn't as tough for me, but I found Ghirahim's final form a challenge. He was tougher for me than Demise just because his battle had multiple stages with no chance to save. First the horde battle with the Minion Red Shirt Army - then the Endless Plunge (three stages), then having to shatter his sword to get to his chest-diamond- a couple of times, was it? Demise was more or less a straightforward fight with a little lightning added mid-battle... it wasn't as complex.
That's what I like about the Twilight Princess Ganon/dorf battle, too - it was multistage and complex. Puppet Zelda with the traditional "tennis," followed by a couple of stages of beast vs beast, followed by a battlefield chase with aiming arrows, then the final swordfight. However, it disappointed by being too easy on the final stage.
When playing Skyward Sword, I felt lit was the TP final battle all over again - cool stuff to lead up to it, only to have a fairly simple, straightforward final stage.
So, I guess what I am saying is that it would be nice for a final Zelda boss battle to be very complex and staged and perhaps have an enormous awe-inspiring final form of the Big Bad.
(Making him a puzzle-boss like the Colossi couldn't hurt, either. SotC is a simple game that makes up for that with having the complexities in the bosses themselves). While Zelda does that to a degree with their dungeon bosses, I think they could learn with Demise/Ganon/other Final baddies.
Vaati in Minish Cap was complex, and while I complained about it on this very board, in the end, when I beat him, it felt wonderfully victorious, to the point where I came to appreciate how many different forms he had.