This is just some fan fiction I had while playing your mod. Hope it tickles your fancy.

Plague
- Starvation or pops in low habitability are perfect conditions for plague.
- Spreads among trade routes and smugglers.
- Plague (a) Affects one species.
- Plague (b) Affects one catagory. (mammals, fungal, anthropod, etc)
- Plague (c) Affects all pops.
(- Plague (d) Intelligent Hive Mind Virus. Infected pops form new species; attempt to seccede.)

Diplomacy
- Close borders to infected empires.
- End migration treaties before there's a problem.
- Research Agreements give bonus to researching a cure.

Policy
- Purging is a viable strategy for dealing with plague. Don't discount it.
- You should probably forbid migration within your empire.
- Refugees may have the plague. Xenophobes proclaim.

Tech
- Mid game tech allows your science vessel to assist research for a cure on sick planets.
- Mid-Late game tech allows you to use biological warfare.

Ethics
- Xenophobic factions and ethics will see a rise in popularity during plague.
- If the cure was part of a research agreement between empires, your pops gain xenophile.