We find ourselves gravitating more and more toward https://realtimeboard.com/ for distributed planning and brainstorming. It's replaced whiteboards for some in-person planning meetings as well. In almost every case where I've said "I wonder if...", it's worked exactly how I wanted. ("I wonder if I can resize a group of sticky notes," "I wonder if I can add a comment and indicate reinforcement/critique," "I wonder if there's a timer plugin," etc.)

It can get a little slow for larger boards – we did multiple weeks of branding iteration in a single board with hundreds of images and sticky notes – but overall I'd highly recommend it.