I'm in the process of revamping my 5th ed. campaign, and am looking at dealing with spirits. Although ghosts and spirits designed as using Desolidification is workable, I was attracted to the spirit rules presented in the 4th Edition Hero Almanac 1 and Horror Hero. I tried reworking those rules a little bit by adding a new primary characteristic called SOUL (defined as a spiritual version of BODY), and a figured characteristic called STY (Sanity - defines as a spiritual version of STUN). I also ruled that MD would be a figured characteristic along with PD and ED (which meant that everyone got a little something).
Using the new characteristics, characters could develop powers that used, targeted, etc. SOUL instead of body. For example, a vampire's life-draining bite could be written as an HKA vs. SOUL (although part of it would still need to be vs. BODY). Given this, MD would need to be able to use things normally only available to PD and ED (like rPD and rED). So MD could have rMD and MD Damage Reduction. Although I'm still kind of partial to the idea, I would need to do some serious tweaking of templates for my HD and HCM software I'm planning on using for the campaign.
But I thought I'd throw this out to the forum to see if anyone else has other ideas on what to do. I had been picturing planes that prevent anyone in physical form from entering (spirit planes, dreamlands, etc.). So only spirits, ghosts, characters traveling in astral form, and the like would be able to enter. Any suggestions or ideas? What approaches have you been using?