Here is what Meddler had to say about Sejuani's rework, what details they've more or less settled on, and what they are still experimenting with:

"Hey
folks, first update on progress with Sejuani here. Since the original
post I've gathered a bunch of feedback, particularly through threads
like this, and tried out various changes in internal playtests, some of
which have tested well, some of which haven't and some of which are
still works in progress. The goal of these changes is to try and
maintain what people like about Sejuani while adding a bit more
perceived impact/readability to her abilities, increasing her
opportunities for big plays (and, if needed to balance that, adding a
bit more counterplay in exchange) and investigating a bit of increased
use of her boar/flail as thematic elements if appropriate. 

Examples
of things that have already tested well and are likely to be part of
her rework (generally low risk quality of life improvements): 

1. Changing
   the mark for Permafrost from a very weak slow to a non CC debuff. This
   allows allows the mark's duration to be extended to a longer value than
   would be appropriate for a slow and stops the mark from having a reduced
   duration against targets with CC.
2. A clearer particle for that mark so it's really clear at any point which enemies will be affected by Permafrost's activation.
3. Slightly
   increased responsiveness on Artic Assualt when it comes to detecting
   collisions with enemies and calculating travel through terrain, courtesy
   of some tech improvements since her development.

Examples of things still in testing (being experimented with to see if they work, not guaranteed to happen): 

1. Base stat tweaks
2. Adding
   a bit of tankiness somewhere on the kit to better match Sejuani's
   thematics (tough warrior on a giant boar) and reduce the amount of
   damage she takes early on.
3. Changes to Artic Assault's behaviour on hitting an enemy champion (e.g. stronger CC than just a slow)
4. Whether Northern
   Winds should apply the mark for Permafrost and then have its gameplay
   tweaked (since the current 'bonus damage against Frosted targets'
   interaction would then be irrelevant)
5. Increasing risk of play slightly and in exchange increasing reward for good play to match.

The
next step design side is simply more testing and iteration, both on the
examples listed above and some other stuff. Then, once we've got a set
of changes we're happy with, we'll refine those, balance them and, where
needed then work with our art department on new visual/audio elements
(e.g. the clearer mark for Permafost as above). As previously mentioned
there's also some interest internally in tweaking Sejuani's appearance a
bit. That's not my area of expertise so I'll try and catch IronStylus
at some point and see if there's anything concrete about that can be
passed on about that. As far as timeframe for all of this goes there's
still a fair while to go - I expect we'll see at least one other rework
out before Sejuani's. Stuff is progressing though and I'll update again
in a while once we've got things more locked down design side."    

http://www.surrenderat20.net/2013/01/meddlers-update-on-sejuani-rework.html#more