A couple of things I found to be of value in my project that you might find helpful:
1) Endpoint-level granularity
A lof of APIs are RESTful these days, and the trend is for more to be that way than not. It would be nice to see (optionally) the uptime broken out by resource for that API. APIs generally connect to a bunch of different backend services on the API provider's side (think the difference between SMS and Voice with Twilio), and there's a high likelihood that one part could be down while the other is not (e.g. SMS is down for Twilio, but Voice is not).
2) Response Times
This is tricky because response times can be fickle for any number of reasons like network issues, for example. However, I've found that there is value in monitoring response times and big jumps (e.g. anything 1-2 standard deviations off the norm warrants an alert). What would be really cool is if you can monitor response times across various AWS regions.