I'm working on a template or two solely for undead (any sorts). Including Fungal (said zombie, lich, whatever, knowingly or unknowingly allowed a fungus to grow on it, sort of a bizarre symbiotic relationship)
Kind of makes me think of the Symbiote in Scarred Lands Creature Compendium 2 I think, the Symbiote that carried with it some ancient magical lore. Even though, I gather the one you are thinking of is a little bit more sinister.
I think willing exposure to fungus & disease is a fine concept, it was actually a small part of Necromancers Legacy, the Anam'Glac eat the flesh of the dead, in order for them to gain some of their powers. In return, they suffer easily from diseases brought upon them by the dead flesh. Even though, the "fungus" you are thinking of is clearly a benefit in itself, not a side-effect. I would not at all mind seeing what you come up with.
The other being an Alchemical template, said undead has had many many chemicals pumped in, making it a rather unpleasant thing to try to kill.
I was actually working on a magical item, a huge, glass cylinder which could contain the life essence. The essence is extracted through a horrible, macabre ritual after which it is stored in alchemical liquids, which are stored in the huge cylinder. Undead creates can then be attached by rubber tubes and iron manacles to the cylinder, forcing the vile liquid into their veins and into their system. The undead would have some requirements before they could be affected though, they would need to be corporal of course, but would also need to be in relatively good shape and have their blood vessels and flesh intact.
Also one template I am building (faster than the other two it seems) is that for the Boneless, from an old issue of Dragon (not sure how helpful that is, issue 233 or so I think) Zombie who had it's skeleton animated basically.
Here Im hopelessly lost, I havent got a single issue of Dragon. I wouldn't at all mind seeing what you make though, send it to me through email when its done.
Also had several when I was trying to make my own Necromancy book, which got lost along the way (with all the ones that hit the market) A lightning conducting skeleton, a zombie that grows it's own weapons, a skeleton with a petrifying gaze attack, and a form of undead who were creatures who spent too much time on the Negative Material Plane and got all their life force sucked away.
Well, its hard getting started. I had been thinking about it for some while, and finally got some friends talked into helping out. Then the good Jason called on my Mail box, which is why its now going to be a published follow-up of Necromancers Legacy.
I actually think I have located all of the above mentioned undeads.
Lightning conducting skeleton: There was a post regarding a home-brewed Lightning Zombie in Creature Catalog. You might want to check out the Lightning Powered Grafts comming up later, they could give a little of the same spirit

Zombie that grows its own weapons: See the "Undeads" book. There is actually a Template, I think its the Risen, that allows a creature to grow its own bones into usable weapons.
Skeleton with a petrifying gaze: Hmmm, have you seen the Alabaster maiden in the original Necromancers Legacy? Undead with a gaze attack...
Last one I couldn't find. I normally develop monsters the other way around, thinking of some background information and description before I invent the abilities. Normally, I only create creatures that already have a place in the campaign or history, before I even considering their stats.
Also, a thought on spells, how about one that animates Outsiders as undead? As I recall, the first Necromancers Legacy (which I still haven't got a copy of has an undead Angel, so I don't think this is a far stretch. I'm not good with designing spells though so that one could be someone else's puppy.
Well, the Angel you are thinking of is the Crypt Angel, and its actually still an Outsider, it hasn't become undead. I myself think that being an Outsider prohibits you from being animated as undead, even though I know Upper_Krust has some undead deities template in his new book. An undead deity would, in my opinion, be a combination of Outsider & Undead, but I have still some difficulties getting around the concept of an outsider undead
