Blizzard gave us a handy overview of the new 1-5 player 10-15 minute Delve mini-dungeon feature! From their role-independent nature to their tier difficulty levels, the companion NPC and his specialization and curios, the 12 unique Delves (and the special single-boss 13th one), Delve variants, rewards (going up to Heroic raid level/M+5 keys), the customizable mount, and more. 

Here's a preview of Arathi Lynx mounts coming in The War Within expansion.
The mounts currently have no source indicated and come in various tints as shown below.
Black

Brown

Gold

Red

White

Warlocks will be getting quite a few changes in upcoming Alpha builds as well, as Blizzard give us an overview of talent changes.
Warlock (Source)
Greetings Warlocks,
We’ve been very excited to see the community’s reactions to the Warlock Hero Talents, and we appreciate all of your feedback.
In the coming weeks, you’ll see multiple changes to the Warlock class and specialization trees, and we’d like to provide some context for these changes here.
Core Goals
Ensure each capstone is interesting and exciting. Reduce the amount of throughput talents, to free up utility choices. Reduce the number of ranks for talents that don’t warrant multiple ranks. Affliction
The Affliction specialization tree forces a choice between a single-target or multi-target profile rather than providing a viable build that is adaptable in both situations. We want to redesign, introduce, and remove talents, as well as address the tuning of Malefic Rapture to help reduce this friction.
Demonology
For Demonology, we want to address the number of long cooldowns available as well as reduce the amount of resource flooding. We’ll also look at underused talents such as Doom. The feedback provided during Season 3 was instrumental in helping illuminate a path forward for Doom, and you should expect a redesign of it in a future build.
Destruction
For Destruction, we have plans to reduce the fluctuation in power based on target count. We also want to take another look at how Destruction plays in multi-target situations and look for ways to improve underused talents such as Cataclysm and Soulfire.
More to come
As always, we’re reading feedback and discussing other changes. We’re still monitoring discussions about Hero Talents, and will make adjustments as needed.
Thank you for your thoughts thus far. We look forward to hearing more as development progresses!

Blizzard have detailed upcoming Mage changes on the Alpha, featuring simpler rotations, changes to talents and their flow, the removal of Temporal Warp and much more.
Mage (Source)
Hello mages!
We have some updates coming to the Alpha in the next few weeks and want to give you an early look at a significant talent shakeup.
These changes will be almost exclusively specialization tree changes, but the goals are similar across them all. In general, we’re happy with the core gameplay of each mage specialization. These changes aim to elevate existing gameplay, provide choice, and ease frustration.
Goals
Allow you to acquire each specialization’s core gameplay with less talent point investment. Simplify rotational complexity. Provide tools to help you adapt your damage profile to better match a given encounter. Provide competitive choice nodes that let you opt-out of complex gameplay. Mage Class Tree
So far there is one change in the Mage class tree:
Temporal Warp is being removed. Temporal Warp has some experiential issues we aren’t happy with, like mages not being able to use it in raid fights where Time Warp is not used at the start of the fight. We’re also unhappy with the significant APM requirements it creates, and how failing those might snowball into substantially reduced damage over the course of a fight.
Mage damage overall will be tuned taking Temporal Warp’s removal into account as these changes roll in.
Arcane
The Arcane spec tree will be receiving updates aimed at providing more options for players who enjoy the fantasy of Arcane and want to play it without some of the demanding elements of its current playstyle.
Arcane has a lot of inherent complexity with mana management, and that complexity is not well represented in the spec tree. Talents that might seem simple to manage on other specs become a headache to juggle alongside Arcane’s burns and mana management. This, paired with the new elements being added by Hero Talents, make it seem like the right time to address these issues.
We’re still working on the changes, but we can share this: Radiant Spark will be redesigned to be much simpler. It will also have a competitive choice node for people who do not like the scripted style of gameplay it incentivizes. We’re also looking at increasing the power of Arcane’s procs and giving them more exciting procs, such that responding to them as they come up is almost always the best move, even in your burns.
Fire
Fire will likely be the first to test new changes. These changes are targeted at giving Fire more talent options, particularly in AOE. We want you to be able to assess the encounter and adjust your talents accordingly, and that means being able to alter how good Ignite/Flamestrike/Phoenix Flames are at different target counts. We’re also taking a look at Fire’s suite of maintenance buffs and culling a select few of them.
Frost
With the exception of a few talents, which we’re looking to address, we’re feeling good about Frost’s gameplay and tree. Our first batch of changes for Frost will be small in scope, and targeted at adding a new build that should reminiscent of the Deathborne playstyle from Shadowlands, while also freeing up some talent points.
Thank you for your continued feedback, and we’re excited to hear more from you in the weeks to come.

We have yet another interesting set of mounts coming with the next expansion, and while they're not... traditionally pretty, they sure are cool! The cave grubs are another ground mount in an expansion that is really focusing on flying, with the Dynamic Flight expansion of Dragonriding, so we really hope that rumored dynamic ground mount system comes in sooner rather than later. Could this be another burrowing mount perhaps?
More cool looking Mounts from The War Within Expansion in #worldofwarcraft

This one looks Nerubian inspiried, which is very cool! pic.twitter.com/SZRY6rdg2Y
And here's the mount special animation, courtesy of keyboardturn: