[@Simple Unicycle] The biggest thing that happens is the physical and spirit worlds sort of merge during the harmonic convergence.  How much that's going to come into play during this rp is up in the air but it is now canon that Republic City has 'spirit swapms' that cannot be permanently damaged or removed.  They're sort of like mini spirit worlds where spirits live.  That didn't really have a big effect on the rest of the series, but the vines from those swamps came to be used as an energy source.  That being said, this rp is going to have something akin to nuclear technology since that's what spirit vines basically are.  Korra's other adventures are likely to be mentioned throughout the rp but most of the stuff that will happen will be self-explanatory.  Having a character who is good with marksmanship and engineering is fine, though I assume he wouldn't be using something directly equivalent to modern guns.