Before you move on to assassin rework, there are still some mage issues that could use a cleanup
: why mages ability to deal with tanks and bruisers depends on whether or not they have dot abilities? Why their main HP-busting item has flat penetration? I understand HP on such item, to discourages Ziggs and other Luxes from buying it, but I see no balance reason why say a
deserves better tank-busting than say 
: changes to this item didn't hit the goal. It's still utter crap on some, and BS pain to deal with and balance terror on others like 
: despite, and maybe because, of great amount of "protobelt OP" whining even before it's release, it's a noobtrap. Right now best protobelt user in the game is AP Rango.
is so useless I almost forgot to include it on the list
: "snowball item" that should be a high-risk, high-reward purchase, instead most commonly used as disgusting cheese-strat on super-safe champions like
or
. Bonus MS making them even safer makes it even worse.
: best AS/AP item also being best AP/CDR item makes champion using all it's stats well like
,
and - to much lesser extent, tho -
permanent balance issues
Now, while I don't expect Riot to make another big mage patch, I sincerely believe that many of those issues - if there are players who agree with me - could be solved with tiny changes, included to the game one-by-one. For example,nerfing Mejais to grant bonus MS only when moving towards visible enemy champions would easily fix abuse-cases, allowing it to be strong on champions it's supposed to. Granting champions period of immunity to Rylai after being slowed several times in too short window would allow it to be buffed to the point when it's reasonable buy for a mage trying to boost their utility, without making it disgustingly strong on others.

, and to a lesser extent,
.
. so whats left for tanks? bruiser builds like
to deal damage to burst squishies instead of building full tank and working off your utility.
they work more like carries than usual mages.