I want to be able to white label status pages, email alerts, etc. (total white label possibilities)

It would be useful to have bulk editing options for the "check interval" and "monitoring locations".

Show somewhere on the status page, the SSL Labs grade for the monitored host. It can be get trough the API, like https://api.ssllabs.com/api/v3/analyze?host=updown.io (it could be tested once a day for each host)

Google Sheets integration.
We use the API now with an external Python script polling in data every x minutes to keep our overviews up to date.

I don't know why, but I always get confused if the far right is the current hour or the far left is the current hour. A mouse-over, tool tip or guide that just says "current hour" or "24 hours ago" would help clarify at a glance.

I believe the effort to turn you application into an webapp is pretty low (just include the manifest.json into the project root), and that would enable the users to install the website and use as an app.
It would also open some doors for future development:
- Send push notifications from web
- Offline access (yeah, this is kinda weird for you product)
- Improved caching
- Auto-updates straight from web (this is in comparison to native apps)

I would like to have checks which fail when something is there which shouldn't be there. For example a http endpoint which should only be available through https.

I like ordering checks by status having the "down" first.
It would be nice, in this view, to have the disabled checks at the bottom of the list.

I'd like to be able to enter multiple email addresses for the weekly report. For example, certain department managers would like to receive a weekly report of the checks, but not be disturbed every time a check goes up or down.

We would love to be able to post alerts to Google Chat using the incoming webhooks (cf. https://developers.google.com/hangouts/chat/how-tos/webhooks). A similar integration to the one already existing for Slack would be wonderful!

On the dashboard it would be nice to see the timing to the first response

To save time on bookkeeping, add UBL invoices.

It would be nice to be able to choose which notification recipient (email, sms, ...) to activate/deactivate when creating a check through the API.
For now all recipients are activated by default.

Allow maintenance window for planned IT job (server down)

Right now I have about 20 subdomains across 3 top-level domains.

The current "domain-suffix-oblivious" lexical ordering is kind of a mess.

I really wish all my "hexstreamsoft.com" subdomains were grouped together, and same thing for my "hexstream.net" and "hexstream.xyz" subdomains separately. Right now those are all "randomly" jumbled up together based on the lexical order of their lowest subdomain components.

Provide integration for https://www.pagerduty.com/

It would be nice to have either page specific or account wide custom stylesheet applied to the public status pages. Specifically, I'd use it to draw more attention to the translation dropdown and to eliminate some whitespace.

Pretty simple, when you add a webhook there should be a "test" button which fires an example event to the webhook URL so the user can test it to ensure it works.