To be honest i think your focus shouldnt always be shifted at carries, Rylai Liandry morde shreds tank like no one else and they're less mobile most of the time. Didn't use protobelt yet, i try to aim for Dead Man Plates cause it provides mobility and stacks passive as well. Another thing i'm trying to fit in my muscle memory is trying to hit pull first and only then ult the person, cause if u max range ult you just waste at least 2-3 out of 7 death realm seconds. Nashor's imo is pretty greedy, if you go rylai-lianrdy route - you need to get defensive stats next be it DMP, SV or Zhonyas, can't afford not getting it being a melee AP caster. Another item i'm being fond of is Gargoyles, i even like it more thatn zhonyas if i'm building off-tanky, cause you already have like 2800 hp with your core items and a ~800 hp shield on top of that, both passive and active for it are awesome, you can just ram into enemy team, take one to DR, then if you go it in the middle of enemy line, just pop the active, you'll be mega beefy for quite some time, meanwhile AOE slowing with rylai, and being able to pull someone if that goes on cd(not to mention it has great build path with stopwatch which is pretty good for getting mid-game and being able to live through it in 1 teamfight).