I would like to see "Athleticism", though it would not really work as class, more as world skill line. Perhaps one you might need to unlock by finding a hidden trainer or something...

In any case, adding new classes would vex a LOT of people, so any ideas for new skills or new passives are beter served by thinking about implementing those some way that allows people to "refit" it to existing characters - once that's possible, its everyones personal choice where they put their skill points...

But to get back to the OP... I for one like climbing to places, whenever I have a cliff block my path, and time to spare, I try to find a way to get through it, whenever I see some off the beaten track land, I see if I maybe can get there. Often encounter those dastardly invisible walls... sometimes a jump that looked good in theory proves less feasible in practice... but sometimes I manage. Fun times, when one finds an accessible area that's bereft of shrubberies (the knights who say ni would despair) because noone was sopposed to get there... (and no, I don't do these explorings on my sorc. Streak is cheating)

And some of my characters like to travel "per pedes" instead of ride. (despite having a maxed steed - its a RP thing... okay, NOT in cyrodil, or rather... not when I am trying to get to a siege before its all over and done with, or somewhere other in a hurry. Though when I have the time... and nothing planned... I often take the long way, and enjoy the scenery, even there...)

Personally I'd also love more athleticism then that... a rework of other activities, like swimming and jumping - if running costs stamina, then so ought it be tiring to jump around like a rubber ball, or to swim. Especially if you swim in your armor (well, lighter armor, in full plate you ought to sink like a stone...).
Implementing those, and then offering somr passives to migrate those costs in an athleticism skill line... I'd love it!