You are free to include ground textures and shared products, as long as you don't modify the mods themselves.
You may add additional unlocks, but please don't change balancing if that's not the core theme of your mod (e.g. complete rebalance mod is OK).
Easily add shared mods by adding the following URL pattern to the dependencies in modinfo.json
with ModID
as the name for the zip file:
"ModDependencies": [
"https://github.com/Drakkam/Anno-Mods/releases/download/Marble_Slabs/Marble-Slabs-Drakkam.zip"
]