@Jolenealaska's focus on hydration is dead on.
The recipe appears to produce about 2 lbs. of dough @ 67% hydration (depending on how you scoop/pour/sift your flour to measure). Your substitution or addition of ingredients should aim to maintain similar hydration, weight and volume (for use in a bread machine).
The best way to determine hydration percentage will be by using a scale to weigh your ingredients (simply stated, weight of water divided by weight of flour).
You're going to need to experiment a bit (not a bad thing... more bread!), and here's some feedback based on your original recipe above, the "other ingredients" listed and personal experience. Adapt as you learn more.
Seeds - most seeds can just be added without a problem. Too many seeds can interrupt (slice through) the gluten structure. Some seeds (like Flax) should be toasted and 'popped' or ground before adding (otherwise they don't really get digested and you miss out on all the good stuff - research this).
Toasting seeds is just plain recommended if you're going to put them in bread (personal opinion).
Corn Meal - substitute one cup of corn meal for one cup of flour and ADD two to three more tablespoons of water.
Oat/Wheat Bran - I pre-soak whole bran for a few hours. Whole bran is thirsty and typically needs about 1/3 (33%) more water than flour and time to hydrate. This is where the scale is handy but the math may be a bit tricky.
You want to maintain the same hydration level for the final dough, but the whole bran needs a bit more water, what to do? Mix equal parts (by weight) bran and water. Then reduce your recipe by the same weight in flour, but only by 67% of the water used to pre soak the bran.
Example: If using 4 ounces of whole bran, pre soak it with an equal weight (4 ounces) of water (67% hydration plus 33% more = 100% water).
Then reduce your original recipe by 4 ounces of flour but only reduce the water in the original recipe by 2.7 ounces (about 5 Tablespoons). Why reduce the water by only 2.7 ounces? Because 2.7 is the effective hydration (67%) of the pre-soaked bran.
Multi-grain cereal - I have no experience with this.
Have fun and enjoy all the good bread.