This is something which would be massively useful.
I have recently considered implementing a DIY email template solution, as I now have about 90 email bodies to edit when I want to add something to most alerts. This is complicated by the weird method in which the body seems to be stored in the database - I can't see any newlines, for example. This could just be me not understanding it correctly, though.
It would need to be done in such a way that template content could be added to individual alerts, rather than just having a "use this template for all node alerts" method. For example, adding a [::StandardNodeInfo::] template would add all standard note information to an alert - address, IP, rack# etc.
A preview feature would also be useful.