I don't really know what to expect in terms of changes or additions for ninja in the next expansion, but I know some things I would like to see.
Currently I think ninja feels really good. It's got a lot of things to do and maintain. I think that what it could use would be some small quality of life boosts. Some of our passives seem extremely worthless. "All Fours" reduces our falling damage by a small amount, but I've found that in almost all dungeons there isn't even falling damage. I can't think of where in the game this could really be worthwhile. Then "Fleet of Foot" has a really meager movement speed increase. This movement speed increase isn't even taken into account while sprinting. Ninjas sprint at the same speed as any other class.
I think that it would be nice if the "Fleet of Foot" ability's movement speed increase was another couple % and affected ninja's sprint movement speed as well. Maybe even consider letting ninjas run on water while sprinting just for fun.
Then I think it would be nice if "All Fours" was removed and replaced with a passive that allowed our ninjutsus a chance to cause an effect. (20%~ Chance)
Like give Katon a chance to apply a 9 second burn.
Doton a chance to slow the target for 3 seconds with each tick of damage.
Raiton a chance to stun.
Hyoton a chance to root.
Suiton a chance to increase damage to the target for 3~ seconds.
Hyosho Ranryu a chance to bleed the target for 9 seconds.
Overall just add some RNG to spice things up. I enjoy how ninja has to keep on its toes a bit as is. With these additional random effects going off it could add some more fun to the class.