Disguise is a unique skill that is currently underutilized due to a lack of gear and a lack of classes that can invest in it. I personally think the concept of a stealth ability that more so literally hides in plain sight is far more interesting than whether you meet a passive stealth/spot check.
Spot currently is very powerful as a skill, as in comparison to other skills, Spot counters two different skills: Disguise and Hide. I won't get into the Stealth versus Spot debate in this thread, and try to stick to Spot versus Disguise. While I recognize that we try to stick to PnP when possible, I think that Spot is balanced (both in terms of gear, feats, and availability as a class skill) towards countering Hide as a skill and just overkill when it comes to Disguise. I don't mean to turn this into a request for Disguise gear as I'm sure the Developers have a lot on their plate with the other changes being made to the Hak pack, but there is a gear discrepancy for Disguise that makes it that much easier for even undedicated spotters to see through disguises.
I'd like to propose three changes: firstly, I think that Search as a counter to Disguise would alleviate some of the issues Disguise faces as a skill. Namely, it makes it so that Spot isn't as powerful as it already is in countering two skills. It also gives more reason to invest points into Search. I feel as if it also makes more sense, headcannon-wise, to search someone to see if they are disguised rather than having x-ray eyes that can penetrate a helmet/mask and a different outfit as well as likely use of the Disguise Self spell (which I believe technically requires another check to discern in PnP regardless).
Secondly, I'd suggest that other classes get access to Disguise as a skill. As of now, I believe the only base classes that get access to Disguise as a skill are Rogue and Bard. The skill is predominantly had by PrCs: Assassin, People's Champion, Shadowdancer and Shifter (I could be mistaken if I'm missing one). Two of those, Assassin and Shifter, likely don't have the skill points to invest in these skills. People's Champion is also incredibly rare, and I feel like a build for one would likely not have the points to access disguise to any meaningful level. Sorcerers and Wizards, for instance, typically have more skill points than they have actual class skills. My suggestions would be that a couple of other classes can get access to Disguise: Sorcerer, possibly Wizard, and Druid get access to Disguise as a class skill.
Thirdly, to put Disguise on par with something like Hide, Spot and Search, I'd suggest possibly some additional feats that would be equivalent to Stealthy or Alertness (minor +2 or +3 to Disguise checks).