A maybe controversial comment: I'm for the abolition of the concept of coin-days entirely. That would, obviously, mean to follow the path that NXT and Peercoin clones like Blackcoin have taken, and for some may mean "throwing basic Peercoin principles". But afaik research on this topic, by various parties, has shown that coin-days is doing more harm than it is protecting the chain - because attackers, with this mechanism, can always accumulate more power with less stake and once they've accumulated enough power to be near an attack, can only be stopped with immense amounts of "staking coins".