Enjoy your repetitive strain injuries, everyone!

The league is called necropolis so I would think that the new expansion will be undead themed.

The voice speaking in the trailer is maybe a new character. The voice sounds sounds similar Cadiro Perandus and mentions prosperity. Cadiro was removed from the game in 3.16. In the lore, the Perandus family is very wealthy merchant family.

At the end of the trailer, we see 5 persons. The winged figure on the left is Catarina aka Mastermind, the final boss of the Immortal Syndicate of Betrayal League. The winged figure on the right is Lycia, the final boss of The Forbidden Sanctum of Sanctum League. The 3 shadowy figures are maybe Ulaman, Amanamu, and Kurgal, these are 3 bosses of The Abyssal Depth.

It is unclear whether the 5 figures at the end will be incorporated into the new league mechanic or if it is teasing a rework of the old game mechanics.

This was the original youtube thumbnail when the video was uploaded. It's still cached in the youtube API. The image shows us a closer look at the man who performs the ritual at the start of the teaser.