You could make any progression you like, and could even make it where different martial class get different numbers of boons and at different times.Would something like a boon awarded when casters get certain spell levels work?
At most, maybe 6th, 7th, 8th, and 9th? But that might be too much, LOL!
I was just rereading a lot of the boons, and frankly a lot of them are exactly in line with ability ideas we discussed earlier. But probably the biggest benefit, it lets the player and DM tailor their level of mythicness. There are epic boons that are awesome but still fairly mundane, and their are boons that are completely supernatural, and its completely up to the player and dm where they want to go with it.
The more I consider this idea, the more I think its a really great way to go about it.