That would end up being a +40 bonus to disguise on top of the spell's +10, right?
A high level bard with skill investment in Disguise, Perform, and Influence could easily exceed 100 Disguise, if this change were implemented, which would dwarf the meager +2 to DCs that the feat gives.
By holding an apprentice illusion staff (which conveys Spell Focus: Illusion) you wouldn't even have to invest a feat to gain it. Gnomes get Spell Focus: Illusion by default at level 1, additionally.
I think sticking to the spell as written in PnP is the best, here. The only part of its OG function that we can't replicate on the server is that when someone touches you while you're under the effects of Disguise Self, they get a Will save to see through it altogether.