I love the theme, but although this is unpredictable as heck, it can also be incredibly powerful. I'm thinking mostly of Fire Shield with 1 DR (holy ouch) and Diamond Shield with 4. Due to Fire and Earth creatures being very good, it'll happen somewhat commonly, yet I can't see this shield in anything but MA domination (with nice speed) and rainbows (where you'll have repeatable CC in order to influence what your shield will be).
Very interesting, not sure if balanced. Without the +1 DR, I wouldn't be worried, but Permafrost Shield, Skull, Fog, Dusk, Tower, Spine and Procrast with +1 DR is very very strong. Most definitely deserving of a cost raise, as 2 DR Hope or Mirror is strong too. Only Diss remains fairly useless, UNLESS planned for.