i would suggest that it would keep the current 400% spirit requirements BUT it takes the caster's spirit into account or the caster's intellect which ever is higher and uses that number IF it's higher then the target's numbers of the same value
this makes it so that if a feral druid used this on a paladin (not much spirit)
it would quadrouple their int as mana regain
if it was a resto druid using this on a hunter the spirit or intellect (which ever is higher) would get changed to MP5 and on target would regain mana it would be a complex thing to work out how to use but it keeps the pvp and pve variability without running the risk of being out leveled
also could be fixed by giving innervate ranks..
such as 450% then 500% etc
considering at between 40-60s ur base mana is most of ur mana druids would be able to clear cast essentially for 20 secs with this change