[calmon]

My solution would cost some coding time:

Gold/Resources don't change, but each sacred glamour unit costs 1 additional Air Gem to build up.

You've something to invest at the start and can't build up as much units. Later you can spend your air gems for a bigger sacred glamour army.

That would give Vanheim/Helheim something special and if you don't play a bless tactic you can use your other non-sacred glamour units (without gem costs).

2-3 Air gems/turn should be provided to vanheim/helheim in each era.