I think this is a great idea. I don't like it when corpses are just left around.

Great idea indeed.

As early as Paleolith it can be a simple grave, corpses decaying by the hearth amid the camp for half a year is a bit lame even for that age :)
In Mesolith perhaps burying tools and grain with the body should be required (and affect morale if can't be afforded or denied by the player).
In Neolith one should probably build some stone structures to room the dead and things to be buried with them.

Also, there probably should be some social hierarchy within the tribe: the chief, the medicine man, a group of best warriors and/or hunters, a group of wise elders forming a council of sorts (groups probably being a fraction of the overall population or their age group, e.g. 1 in 10 or 1 in 5) who would requre different ways of burial (and different amount of things to be buried with them) depending on their social position.

After all, Homer's "Iliad" and "Odyssey" with all the kings and heroes there are about events somewhen on the border of copper and bronze ages. I haven't re-read these things in ages, but I believe they contain descriptions of how they used to bury their dead.

Needless to say, diligent compliance with proper burial ceremony (which should be optional in case there is a hard time of sorts that doesn't allow it) should be rewarded by morale boost for religious reasons as the people get reassured in their righteousness. Failure to comply with the ceremony on the other hand (e.g. someone was dragging a magalith from the other edge of the map and froze to death when winter came and/or got eaten by lions, or was struck by a lightning while in water and drowned, or there simply was too many dead due to catastrophic famine / plague / raid, etc.) should result in them feeling bad about it and getting morale penalty.

Finally, the dead raiders could just be piled up with some straw and sticks thrown in, and the resulting heap just set on fire for good.

I do wish something happened with corpses. They need buried or leave behind bones (that one I'm surprised isn't in the game, humans leave bones same as animals! Might as well let ol grampy Darok live on as a spear or something!) or you can build a pyre or they increase disease chance if left unburied. Or having those viking fire rafts or something. Just random thoughts.

I hope the devs do something with that. It's a cool depth adding idea.

We could also add human sacrifice :D