Adding extra ingredients to the dough

There are many ingredients which may be added to a basic bread dough or various sweets (e.g. fruit cakes) for enhanced texture, flavour, and nutrition. Examples include: dried fruits, grains, seeds, nuts, and legumes. 

However, it's important to consider how these "add-in" ingredients will effect the dough:

They tend to weigh down the dough, thus usually reducing loaf volume. 
 - this is mostly an issue of expectations; the more add-in ingredients to dough, the denser the finished loaf. It's a balancing act depending on the desired outcome. 
Recipes abound with how much to add-in, but really it's a matter of personal preference and budget, then testing! 

They can damage the gluten structure in a developed (mixed/kneaded) dough. 
 - this is primarily about controlling incorporation. Most add-ins will 'cut' through the developed gluten strands, so they should be incorporated lightly, quickly and gently.
Most add-ins require only a minute of low speed machine or hand folding through at the end of mixing. Often, more than this would negatively break down the add-in as well (e.g. fruits or olives).    

They generally impact the hydration of the dough is some way; some ingredients absorb moisture (e.g. grains), others give moisture (e.g. olives). 
 - this requires adjustments to the dough or ingredients. Moist add-ins can be dried well (e.g. press olives between paper towels) or mix a slightly firmer dough. Dry add-ins require pre-soaking or a slightly wetter dough. 

Extra ingredients can be a wonderful, delicious enhancement. Just remember these considerations for the best result.