I'd hesitate to call that a typical definition of a mummy, but I suppose that would make sense considering the xcom lore. Though if this is the case, maybe zombies would be involved with cult of Dagon? I mean, its a different thing kinda, but dagon works with pyramids, which are typically Egyptian and so are mummies, so maybe these tombs of zombies can actually be researched from dagon people? Would make slightly more sense than suddenly finding tombs because reasons, with no mentioned tracking as far as I'm aware, and I could see a partnership between fish people cults and zombie groups, at least in terms of the ones with higher sentience. So maybe representatives of dagon and mummies could work together...?
By the way, are you going to add inventory items that alter stats like piratez does? This might be a good way to flesh out advanced medicine, perhaps with drugs patches to raise stats in expense of others. Maybe a stimulant to raise reactions and stun regen, at the cost of aim and bravery, a steroid patch to raise strength energy and melee, at cost of health, and a mild relaxer, raising bravery and aim, at the cost of reactions and energy. Just a little thought, this was added a little while ago but you haven't added anything that utilized this ability.
Edit : so apparently getting alien origins is almost impossible till 1999 with the recent ufo changes, where up till the invasion there's no ufos. This means for 6 months I'm effectively sitting around doing research, as many things are lmpossible to get until i get an alien corpse.
I'm not really against lacking ufos, but maybe we could make a mission branch involving hybrids and a sectoid, after researching the alien communicator? This would be more reasonable, i think, than waiting for several months, and would be a nice transition into fighting aliens, having a compound with heavily armed hybrids and maybe a couple posted sectoids, and other stuff.
Besides, seeing how the invasion is planned, but alien operations still happen on earth, it would make sense to have sleeper cells of low level aliens somewhere on earth, managing the hybrids, processing data, and being in charge of operations on the ground. Besides, it might be an interesting opportunity to expand hybrid lore, perhaps something about people being abducted and relentlessly genetically modified till they are hybrids. And it would make sense the network on the ground of hybrids, and the aliens in charge of them, being in charge of the tops of the cults, as they have more direct Intel to the earth and its events than an ethereal across the solar system.
I think maybe it could happen after researching the communicator and terminating a cult, that you get missions which are pretty much just call locations from alien's frantically trying to get in contact with the defunct cult to repair damages done, unaware that xcom has cracked their systems, then invading the compound.
Sorry for such a large suggestion, but I think this change would be a great transition to hybrids and aliens. Also, ufos are incredibly infrequently and hard to catch anyways with promotion 2 gear, assuming they do come early so i feel this would make overcoming that hump more reasonable, even if a compound raid was difficult.