The problem with World PvP in general in Legion is that stat templates don't take effect in world, making world pvp pretty much worthless in Legion. Any mythic raider can walk up to an LFR geared boxer and AoE him twice and hes dead. Its silly. Hopefully they allow stat templates in world like they have in instances to fix this, if people complain enough.

As far as classes go DK's have had a lot of changes. I kept my eye on them since alpha began, then through beta still. If your going DK, you'll still want to run frost/blood os but its about only about half as strong as it was in WoD due to Rime. Your Howling Blast now reguires a proc to do full damage (Buffs it by 300% after you obliterate). You can buff it via talents, and now you have 6 runes to spam it instead. But it wont hit nearly as hard anymore unfortunately without you obliterating first.

For Demon Hunters they could be pretty decent in tank spec for world PvP. One of the main attacks (and most other main attacks for Vengeance) are AoE/Cleave abilities. One interesting ability is soul cleave or something like that, it hits all enemies in front of you for (on my DH) 158k dmg and heals for 180k. As long as you can manage being hit at range they look good. Maybe not the best damage though if your facing larger raids with many healers, you won't beat the heals unfortunately. The other DH spec increases in speed with Mastery but has very very few AoE abilities. Mainly just eye beam and the other one.

Warriors have had a lot of good work done on them. Arms warriors (If you know how to box warriors, some people find it tough) look fantastic in legion for boxing. Cleave and Whirlwind are basically both main attacks now and Mortal Strike can hit another target. They have a lot of cleave AoE that is just built into their main attacks. Combine that will the CD's they now have and you could run 20 of them into a blob of 60 players and take them all out in a cleave or two. Fury is meh right now, i've been looking a lot at it hoping it improves since currently its the only spec with Bladestorm. But for boxing it doesn't seem amazing. Just my opinions so far from what i have seen on Alpha and Beta