this will be the strongest build for mages after the update
rod of ages and rocket belt and warmogs as core
then extra item choices rylais liandrys void staff ludens zonyas
most champs will want rylais, liandrys, and void staff. making the total build provide insane damage to tanks (rylais and void staff penetration) while making the mage itself very tanky and impossible to kill
and the warmogs provides a nice chunk of tankness with insane regeneration (a level 16 garen passive on a item. its def worth). after you survive the burst as a mage, you throw out your spells and kite back with rocket belt. and throw your spells out. and then as your team deals with the fight your in the back throwing spells and having insane health regen from warmogs
some champs will want zonyas like twisted fate. but for the most part i feel the other 3 items are stronger
also if a champ doesnt work well with liandrys or rylais (such as some champs dont care about having a slow) then you can swap it out for ludens or deathcap
due to the lower cost of mage items. the power of mages will be in getting 6 item builds without needing boots. while ADC items are much more expensive so mages will get 6 items much faster than ADC's. letting them group with the team and take inhibs to end the game while the ADC is still trying to farm
mages with the 6 item build will kill squishies fast, and will also kill tanks fast, while being super tanky themself
also. i feel the best mage mastery will be strength of ages with the +15 armor/mr mastery. giving them even more tankyness making them impossible to burst. if thunderlords or deathfire scaled with bonus hp/armor/mr it might be worth it but since they dont scale it makes strength of ages just better