Mycelium is open for anyone to look at and add features to. The only restriction is that you don't publish your own wallet, and whatever is added goes into the official app.
That's not very clear.
On the one hand you say I am free to add features, but then you say the features I add must go into the official app. What if the feature I add is rejected by whoever controls the official app?
Are you saying I'm free to add features to my local copy of the wallet but not to share my features with anyone else? If so that means it isn't under an open source license, since open source licenses allow users to share changes with each other.