I've been a stepmom for about a year now. Or to be completely upfront and exact, I have been in my boyfriend's, childrens' lives for almost a year. Dad's girlfriend really doesn't do justice to my position in the kids lives. I never thought much about the term stepmom. When I found myself becoming one I defined it as, at dad's house I "step" in and fill the mother role. That is not actually where the step comes from, if you want to know the actual origin you can try to dissect the definition found in the Online Etymology Dictionary.
Some stepmoms opt to use the word "bonus" instead of step. This I don't understand. In a child's life how in the world can one more parent be a bonus? Kids want to rebel and get away with all the things we tell them not to do. I'm pretty sure 3 or 4 parents instead of just 2 puts a major cramp in the ease of parental manipulation children are trying to achieve. They also now have to navigate the complicated and often negative relationships between all those parents.
I do see how the addition of another person can be a bonus, especially one who loves and cares for the child. A relationship that creates security is a bonus. An extra parent, not so much bonus there.
In my family we use the word step to help someone else see our family tree clearly. If someone calls me mom, no one jumps to correct them unless clarification is necessary. The step also helps the kids not feel like I am trying to take their mother's place. I'm not sure bonus would do the same thing.
Bonus just sounds like you are scared of the word step and the realities that come along with the fact that you are the stepmom not the mom. These are your step kids not your kids. Those can be painful facts to face. You have all the best intentions, but you are not an equal parent. You love these kids and you may be doing just as much if not more than their mom, but they don't love you the same way they love her.
You can use bonus, but the biological mom's at the soccer game are still not going to give you full mom credit. Your partner is still going to slip every once in a while and say, you don't understand because you don't have kids of your own. Your "bonus" kid is still going to going to yell, you aren't my mom when he gets mad.
Step is just a word and it happens to be the word used to describe the relationship between members of blended families. Changing the word won't change it's "connotation". I think what really needs to change is your perception of the word and perhaps your expectations of the relationships you are building.