I like diffusion cause i feel like it makes blu more like it was in some old games, blu buffs were mostly AoE and not single target or self target. The only thing i don't like is how it's on a 15 min recast down to a minimum of 10 mins with 5/5 merits, the problem with this is blu buffs are not that great in duration and usually not even worth AoE'ing cause you can't use it strategically in say an exp party cause you can't use it consistently.
since /sch got accession AoE buffs is quite commonplace and blu spells don't work with accession so updating diffusion to be decent again doesn't seem to game breaking to me.
what i suggest is change it so that the initial 1/5 merits is a 5 min recast, and also add in that when a spell is used with diffusion it gets a 5% duration increase then at 2/5 it becomes 10% duration increase and up in 5% intervals to 5/5 giving 25%
This would do a lot for diffusion first off with a 5 min recast it would pair amazingly with unbridled learning, the buff duration increase would make it useful when solo aswell as in a party and would also breathe life into buffs like feather barrier that don't get used much due to short duration. it would also give a reason to merit it more than 1/5 seeing as the 5 min difference from 5/5 merits on diffusion really isn't worth it as it is...especially over enchainment and assimilation.