OK, what if when you are on neutral territory, you can upgrade any card; but with one condition. You must have an equal number, or one less, of at least six(maybe more, I'm just trying to think of the fact that few rainbows use all twelve elements) other elements in your deck(except for "other", because of quantum pillar purposes). This will allow you to upgrade an equal amount of cards for your decks as anyone else, so long as you're using a rainbow deck, while settling the problem I believe exists with TStar's suggestion. It will also allow you to use a mono deck, if you so choose, but will not allow for very much upgrading of it(just as I could play a rainbow deck, but with most of it unupped).