Very cool assets you have here!
They are very well designed and detailed, and are sure to help a dev or two out in a bind. The icons are eye-catching and very mobile game-esque, but they're a little bland to me. I think they could use a bit of a unique touch, one I'm sure will help your work stand out like it deserves to. That is, if you want to be more unique... There's nothing wrong with having a standard icon style. It might actually help, because it can increase versatility and be more open to other games.
I'm trying to think of something that can help this stand out... Maybe creating some kind of motif among the icons, so that it's easy to identify them as your work. I'm sure more asset pack users and makers will stumble upon this thread and give you some advice.
Feel free to check out my game, I could use your feedback: https://wasabiiii.itch.io/grass-elemental-demo-demo
Having options for shadow and outline is a good idea, I never thought of that. Keep up the good work, these look great!