You misread my suggestion whoops.
To make a long story short, the way I see it:
Obviously, it'd track the potency values of the actions you apply, combo values and importantly, dot values would be factored in as well.
You could then apply a modifier over this on a per buff basis. Self buffs and job specific weapon type debuffs would need to be taken into account as you say above, it just wouldn't make sense for many DPS jobs if it didn't. Where things get murkier is of course choosing to factor in party buffs such as balance. IMHO the simplest solution here would be to have it to default to self only with an option to include party buffs into the modifier.
From there, you can display 2 numbers, your current potency per second and the modifier you are on at that point in time. Showing both highlights the effect of good buff/debuff usage which again as you say, is rather important.
If there's a sticky point with going with potency per second, it's on how to handle crit and buffs such as litany/chain. Handling straight % buffs and debuffs is no issue tho.