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:

• Liquid dye (provided it is soap safe and skin safe). 
• Mica (provided it is soap safe and skin safe). 
• Fragrance oils (same). Note: see maximum safe useage rate below.
• Essential oils (except cinnamon). Note: see maximum safe useage rate below.
• Vanilla stabiliser (needed when adding a scent oil containing vanilla/vanillin). Note: see maximum safe useage rate below.
• Color block system (as sold by soap shops). 
• Activated charcoal (not the charcoal sold for teeth though because of other ingredients). Note: coconut charcoal is the one you most likely want, as this will give a shiny finish.
• Oxides sold for soap making, like titanium dioxide. Note: titanium dioxide is what makes a base white. It also is notorious for lessening lather, so use sparingly.
• Ground or whole old fashioned or colloidal oats. Note: you can't use quick or instant type oats, those will go mushy.
• Used very dry coffee grounds. Note: these *might* spoil at a later date anyway. YMMV. Also you cannot use instant coffee.
  
• Turmeric powder. Note: this can stain anything and everything, so use sparingly.
• Ground walnut shell. 
• Poppy seeds.
• Dried calendula/marigold petals.
• Dried cornflower petals. Note: these *might* turn brown at a later date anyway. YMMV.
• Skin safe glitter. 
• Dried loofah.
• Various clays (as sold by soap shops). 
• Honey. Note: see maximum safe useage rate below.
• Shea butter. Note: see maximum safe useage rate below.
• Coconut oil. Notes: added coconut oil tends to dry your skin. Also see maximum safe useage rate below.
• Glycerin. Note: see maximum safe useage rate below.
• Silicon, plastic or resin toys/flowers.
• Water soluble paper. Note: depending on the printer type or ink used, these might bleed. YMMV.
• Temporary tattoos. Note: some of these will bleed immediately or at a later date. YMMV.

Notes:

• For the soap base itself the safe maximum of liquid/butters/oils/powders is 1 flat teaspoon per pound/500g of base (excluding alcohol, as that evaporates). 
• For FO and EO you need the safe usage percentage from the manufacturer. If that is more than 1 teaspoon per pound of base, use the 1 teaspoon maximum.
• For additives like honey, shea butter or coconut oil, it's likely a better option to buy a base with the additive you want already in it.
• For botanicals like aloe vera, cucumber, carrot etc. your only safe option is buy a base that has that is the additive you want already in it.
• Not all additives might be usable/deemed safe if you sell your soaps, that mostly depends on where you live, so do your research.
• Too much "stuff" in a sink like glitter, oatmeal, cornflower petals can be annoying and can also clog the drain. 
• Added glycerin is usually not needed at all as melt and pour soap IS glycerin soap.