There was once a screenshot in one of articles about Sequences (in this article:
https://www.mcvuk.com/yoyo-games-plan-to-inspire-a-whole-new-generation-of-game-devs/ - but it seems that now it's removed). Another article about sequences is here:
https://issuu.com/newbayeurope/docs/gamescom_daily_2018_day_3/10 and there is a small screenshot here.
Sequences (remember, this is not official info, just based on 2 or 3 articles and some mentions here and there, and I may not remember them correctly now) should allow predefined animations of sprites (objects?) in GMS, so you can set
image color blend, rotation, scale, position,
image speed, image index on timeline, and all will smoothly transform (using linear interpolation, or your own path-like tween) all of those values. Also, you can attach events to trigger at each frame of sequence - so it can be used to make cutscene for example. I would say, that Sequences are similar in some way to what Spine offers, but you can get it without separate license and in same IDE, with controls similar than in other IDE editors.
What I hope they add later (in sequences phase 2 or 3), based on official "future consideration list" is: text asset (for both rooms and sequences) and particles (as new type of resource, for sequences and asset layer).
What I would also love they would add to sequences/asset layer, is to change sprite shape similar to draw_sprite_pos, so you can morph sprites to any quad you like. That would make Sequences to compete with Spine, and can add some of 3d-look to some animations. However I didn't spot yet any info that this is coming - but it feels obvious that it will come at some point (maybe from start), as they seem to already support it for Spine (so they already have it in-engine), so maybe it was just not mentioned - there was not too much info about it, and it's hard to say that anything I already found is official or not. There can be also more things coming that are in Spine too, but it's up to their decision - we'll see, roadmap mentions updates of sequences to come for a year, so a lot may be added.
Sequences probably not offer anything new compared to what you can do using GML with a lot of effort (you can find some examples on marketplace for tweens too), but the difference is that you can do it without a line of code and with live preview using Sequences, so it would be much faster and don't require saving data outside of projects in custom format (no arrays, no ds structures, no buffers), and if you makes project in a small team, artists which aren't programmers can work on animations then.
A GML changes could be required on runner side to support sequences in fact, so that's probably why they are coming together.
So yes, 2.3 will contain a lot of good stuff inside