If weather effects are going to be included, then they should be an actual factor in the game.
And I don't mean your character ends up with a few less hit points after traveling for several days if there is a storm.
If there is a storm, then I'd like to think it would show up on the world map.
Beyond this I would like to see fights during inclimate weather affected (i.e. decreased visibility and therefore chance to hit decreased by rain. Decreased movement during high wind conditions. And so on). I would like to see tall cacti (sorry trees but since there are no trees cacti will have to do) which have been struck by lightning, and maybe the occasional mutilated Brahmin.
A radioactive twister probably would be good as a thematic element, but I would like to point out that the likelihood that the radiation would be more dangerous than the tornado is slim to none. The decay rate of most dangerous fission byproducts is such that after a couple hundreds years you won't be dealing with any of it. So really what you are looking at is a radiated land mass which will be devoid of life (the radiation will have killed the native soil bacteria), and contain some long lasting radioactive material, but won't be so radioactive as to present a major danger.
If you are looking for radioactive danger, then a damaged, exposed nuclear storage site like the one they are constructing in Nevada, where tons of material will be stored, would be the way to go. One should note that established medical findings have found that doses in the 250 Rem+ range in one weeks time will require medical supervision, so until you a fairly decent radiation source you won't be expecting any major cellular damage. Realistically speaking you need a gamma or neutron emittor to present a serious radiological threat. Alpha particles are stopped by your clothing, and beta emissions are stopped by as little as a sheet of tin foil. So for someone stupid enough to go rolling around naked in nuclear waste, sure they are going to be in trouble, but with a little precaution, a little distance, and limited exposure time one needn't worry about most radioactive materials. Most of the byproducts of normal decay are beta and alpha emittors, so I'm not fearing for my life barring the existence of large volumes of radioactive material or the presence of something which likes to give off neutrons/gamma rays.