I have a feeling this idea has been brought up before, but I wanted to bring it up in the case it wasn't.
I was thinking of something as simple as just a longer term than the already lengthy 90 days. What about adding a 6 month option and full year option? Larger data buckets would be required, but the overall simplicity of paying for your plan once and not having to worry for the next month, or even year is very appealing!
Just a thought. 😀