Not safe to use additives for melt and pour:

Almost all biologicals, as they will rehydrate in melt and pour and thus spoil. This is a health hazard, whether you can see it or not. This is different when making cold or hot process soap, as then the fresh or dried ingredients go through the saponification process.

So unfortunately you cannot use things like rose petals, lavender buds, dried orange slices, any kind of tea (including herbal tea and matcha), any kind of fruit/vegetable/flower/herb (dried or not, powdered or not), any kind of dried milk powder, ingredients with a high water content (like rose water, juices, purees, gels or milks).

So no to basically any kind of fresh or dried ingredients like that, aside from a very select few, see below.

Also not safe: ingredients that can irritate or burn the skin, like cinnamon, pepper or chili.

Ingredients that will make the soap not set well or at all, like epsom salt, are also not a good idea in a melt and pour base.

Safe to use additives for melt and pour: