You might have remembered the new spells, previously exclusive to wizards of respective schools, available for training at level 100. A natural extension would be to give another at level 110 or 120; here are some of my thoughts about what these might be. These suggestions are kind of based on the popularity of each secondary.
Balance - Balanceblade? Currently few people of another school train past Weakness. OK, maybe some Theurgists have a balance mastery amulet and want a cheap AoE. Since the end of the training line is far past Weakness, there isn't much worth the extra training points to put at the end.
Life - Guiding Light; pretty obvious. Also a stretch from Sprite or Satyr, the latter of which people might be reluctant to get without life mastery.
Myth - A bit popular, but not terribly so, so Mythblade?
Death - Everyone trains Death to Feint, so perhaps Animate.
Fire - Probably Fireblade.
Ice - Everyone trains Ice to Tower Shield, so something relatively useless like Ice Armor.
Storm - Cleanse Charm. Training Storm is completely useless right now, so even this might not be attractive enough.
They said that they are not going to include class defining spells so Balance Blade is OUT almost certainly. Why include something useless? Cleanse charm I think might be popular, however isn't the normal version 1 pip and the TC is zero? Along those lines why not Pierce or Cleanse Ward for Myth? Doing X pip spells like Armor and Animate don't seem to be good ideas. Maybe they could add Fuels for each school? I would also like Guiding Light, but I don't know if that counts as a "class defining spell" or not.