I get Dysport (which is like Botox) in my forehead, for the furrow at my eyebrow and the couple of horizontal lines at the top. I've been doing it about 2 years, once every 4 months. I like it very much and if done properly you still have movement and expression. I've had no issues, like bat brows, and no side effects. I don't tell anyone IRL that I do it. It looks better, but it's not so drastic that my friends are like- oh, botox.
I don't think fillers would be a good idea for the forehead. They work differently than Botox/Dysport. I don't really like the way fillers look in general. I think they're too obvious, and too often look odd.