For Roshar I think I would pick Alethkar and end up in the Ardentia, doing artifabrian stuff for Navani in Urithiru. Seems like the coolest job in the world, reverse engineering ancient artifacts. If I keep my current knowledge I could definitely crack fabrial walkie-talkies and air vehicles.
For Mistborn Era 2 I would want to be a twinborn compounder for zinc, to be the ultimate quick thinker and inspiring leader. Chromium compounding is probably the most powerful of the compounded powers, but I think that much Fortune has Cosmere-wide impact that I might want to steer clear of.
Kelsier is a great character, so solid choice there. I haven't read Emperor's Soul yet (I know, I know, but Stormlight Reread 3 and Wheel of Time is all I have time for given college schedules), but Shai seems like an awesome character too. Dalinar has gotta be my pick though, since he nails the "flawed hero" archetype in a way that feels fresh/clever and fits the story perfectly. Plus, his whole "the next step is most important" thing really speaks to me and what I need to do. Dalinar honestly changed my life. I think as a Radiant I would fit most with the Bondsmiths, and I'm not just saying that since they seem powerful.
So there's my Dalinar essay! It's not my fault I love his character so much, blame Sanderson for that one.