Dual Types

I have heard tell that, early in the development of 5e, the notion of creatures/races with two Types was thrown around, and eventually ruled against. I don't know how it was meant to work at the time, but it does fill a much-needed gap.

Early on in published 5e, PCs could only be Humanoid. The Eberron book made Warforged simply Humanoid, rather than the old 3.5e jerry-rig of Construct(living construct) that made them technically Constructs but also work for PC purposes (mostly resurrection).

Then, later on, they abandoned that principle and started publishing races that are other Types, such as the Fairy in Wild Beyond the Witchlight. They had realized it's not super important for PCs to be Humanoid -- although there are still no properly Undead options (the three Lineages in Van Richten's Guide to Ravenloft are Humanoid, but with various Undead-esque properties). I hear tell there is or will be an actual Construct option (the Autognome?).

My solution to there being no Undead PC options, Warforged being nonsensically Humanoid, and so on: bring back dual Types!

• Some races -- including warforged (humanoid/construct), tiefling (humanoid/fiend), aasimar (humanoid/celestial), dragonborn (humanoid/dragon), kobold (humanoid/dragon), and alithi elves (humanoid/plant) -- have dual creature type.
• If any favorable effect would affect either or both of your types, it affects you.
• If any unfavorable effect would affect one of your types but not the other, you can be affected by it but you have advantage on saves against it.
• If any effect would affect neither of your types, you are immune to it.
• If any effect would affect both of your types, you do not gain immunity or resistance to it from your types.

The one peculiarity here is: what happens if you have an effect that has a positive effect on one of your Types and a negative effect on your other Type? I'm not sure if there are any actual effects that heal a living thing but do radiant damage to Undead, but a character with both the Humanoid and Undead Types would have to deal with it if it does. My instinct is that you are affected by both -- you are, in that example, healed and take radiant damage (albeit with advantage on any saves against it). But this is a rare enough circumstance that I'd leave it up to individual DM adjudication.