Being that a vast majority of my 142 plots and my keep with 100 plots are on almost ALL Tundra it sucks lol That being said the neutral option is an interesting one and feels like it could be the right move. It would also bring some value into natural cards.
However it does seem weird as really those areas should be aligned with a element card and not a non element card.
To me Tundra should be +10 water aka snow and +10 dragon as that seems like a fitting match. I'm not 100% sure on this one though but it would be nice to have a change to put these lands on a even playing field. So for now I'm going to go with the +10 neutral as the option as that sounds the most interesting.
Nearly all of mine are tundra too - hasn't been great. Though I have far fewer than you :)