They don't need legs, and they don't need to rob worgen of Two Forms either. If bipedal races can sit on mounts with their legs on both sides, then naga can sit sidesaddle on mounts with their tails on one side.
Here's an illustration someone made. You can also look at Li-Ming's mounted animation in HotS, even though she's human.
That said, if Blizzard is seriously concerned that people are going to be horribly bewildered and confused over boots not displaying on naga, they
can give them an illusion spell that transforms them into elves the same way we see Azshara do it in Azsuna. I'd rather they didn't, but it wouldn't kill me if they did. They definitely don't
need, though.
I only speak for myself, but I want naga as they've appeared since WC3. I don't want legged sea elves.
Please, no furbolgs.
I like vrykul for the Horde for a number of reasons, the biggest being that they'd help further balance faction populations. They'd likely bring paladins if they were added, so I can see at least a few blood elf players race changing to vrykul, and others might give the faction a shot if they can play as half-giant vikings. A lot of people really enjoy that theme and have been petitioning for vrykul since WotLK.
I'm also sure they're setting up Odyn to die later. If he does, that leaves the kvaldir open for Sylvanas to swoop in and recruit to the Horde. They'd make a pretty cool sub-race option for vrykul, if they're not just added to the Horde instead of them.