midseason mage update is coming but I see no changes planned for Katarina anywhere
Katarina isn't a mage, she's an assassin.
2 different archetypes. Several similarities, several more differences.
She probably has one of the most controversial kits , one being based around exterminating an entire team in a literal blink of an eye.
Katarina's casting times are much slower than the blink of an eye, and her kit isn't that bad.
She's complained about more by people that don't understand the general rules and dynamics of LoL than anything else, striking many as being unfair and game-breaking when starting out.
Once players learn the general rules and dynamics of LoL Katarina becomes a pretty fair, if not slightly weak, champion to play against though. She has solid counter-play, she encourages teamwork, she actually wards, and she has clear tradeoffs in power for her strengths within her kit.
will we have to wait until late-season or maybe even next year to see changes or updates?
Yes, you're not going to see assassin updates in a mage update.
I noticed with the mage update there are no new items being specifically tailored for mage assassins
Mages and assassins are 2 pretty distinct archetypes that really should have different itemization patterns distinct from one-another.
AP assassins lacking good itemization is arguably a pretty big problem right now, with Akali and Ekko giving up on being an assassin and just being a tanky DPS fighter at the moment.
But AP assassins really don't have anything more to do with a mage update than AD assassins have to do with a marksman update.
does this mean that Assassins and AP mages arent supposed to mix?
Not including assassins in a mage update really doesn't say anything one way or the other in regards to that.
With 120+ champions, LoL has plenty of champions that cross several class archetypes, and a few champions that don't really fit into any particular archetype and are something pretty unique. Sudo-mage champions do span a considerable amount of the roster.
It's just not feasible to address every sudo-mage in a mage update though, and simply addressing archetypical magus keeps the update neat, clean, and tidy.
A major focus of the mid-season mage update is also differentiating mages and giving reasons to pick one over the other, which heavily favors going over mages that are average to a fault: and not sudo-mage champions bleeding into another archetype with a strong identity and unique niche, which most AP assassins do have.
on the same level as 
does this mean that Assassins and AP mages arent supposed to mix?
