One thing that frustrates me is that in this day and age, people don't like playing tanks very often... This isn't that much of an issue if at the time I was the jungler or top... or support. But there are times when I am the midlaner or botlaner and my team may lack both magic damage and a tank.
So, what would you pick if you were bot lane or mid lane and there were no tanks or magic damage like in a team with a 
top 
jg and 
support?
MID: 
Bot your pretty much out of luck, adc's aren't meant to be tanky.
I'm not sure about Ranked, but going as a Melee ADC is always fun at bot lane. Yasuo has nice synergy with Janna if you can make it past the less-than-desirable laning phase, Darius can take advantage of the CC offered by the three up there (and with Riven's damage output he can score a nice dunk or two). If you really need a tank, have it be the mid laner because ADCs (especially Marksman-class) should focus on damage and NOT tank stats. For Mid, it's the same but in that case it depends on what your team needs. It looks like your team will be AD-heavy so going an AP champ like Gragas or Nautilus will benefit your team. You could go Nunu as well, especially since his AS buff helps Riven out and his 45% CDR slows are beastly (no pun intended). Darius can also make it at mid lane, because a lot of Meta champs used there have CC in place of mobility -- so if you can dodge or cancel out the CC they're ripe for dunking.
Mid: 
Bot: 
And that's about it.
Theres always 
can eat a ton of damage too
Galio, Cho, or Gragas imo.
Mids : 
not all great picks but can have some balance of magic damage and tankyness.
ADCs : 
You're not really going to get tankyness out of an ADC, but some of them deal significant magic damage to cover that weakness. Except the urgod for the tanky ADC with no magic damage.
But of course your proficiency on a champion will be more important than the pick to fill out the team comp any day.
If I were mid-lane, I wouid probably pick Orianna, Lux, Swain or Lulu.
If I were an ADC, I would probably pick Varus, Ashe or Ezreal.
While having a durable champion is important, tanks are generally valued for their CC due to how powerful initiation and peel are.
doesn't actually need a tank
would prefer a tank but can survive without one to a degree
can countersiege all day while someone splitpushes
strong fox woman and don't need a man, she can go for women and robots
countersiege and siege
become the tank
