"We will eat fruit and bread for breakfast tomorrow."
会 is correct here. But you are also correct. It can be used to express that something will happen, and it can be used to express an ability to do something. 要 is in the same boat. It can also be used to express that something will happen, but also can express a desire or need.
It's the same in English. "Tomorrow we will eat bread for breakfast" vs "tomorrow we eat bread for breakfast". As a native English speaker I couldn't tell you the scholarly reason why this is but if I had to guess it's that it's necessary to put a tense-modifier on the action verb you use. "We will eat", "we are eating", "we ate". In the case of Chinese these come in the forms of 要，会，了，在。Although in some general cases you may not need them like if you say as a general statement "I like to eat food" "我想吃饭"。I hope this helps but I'm not sure how true this is I'm only speculating based on patterns I'm trying to pick up on.