If we understand your question correctly, you are looking for a general replacement for tomatoes in recipes. A puree of red bell peppers with a dash of lemon juice might produce an acceptable result in some recipes. Eggplant might work in others. But the sad fact is that there is no general replacement for tomatoes.

The good news, though, is that there are literally hundreds of thousands of main-dish recipes that you and your family can enjoy that do not include tomato.