I think a custom logo has been suggested before in a comment (https://updown.uservoice.com/forums/177972-general/suggestions/33892252-custom-css-for-public-status-pagess), just wanted to raise it a suggestion now

Add notification escalation.

Add a method for responsible people to aknowledge reception of an alert. Add an escalation of alerts in case someone did not respond in due time (15 min, or parametrable).

Ath the moment SMTP ports may be checked for responsiveness but not for valid connections. Interesting would be:

• check if there is really a SMTP server
• check for STARTTLS
• check for valid certificate / expiring certificate
• check for MTA-STS/DANE

Allows to monitor WebSocket only endpoint (ws:// and wss://)

When using a service such as StatusPage for client updates, it provides a long unique email address for each type of alert we need to monitor. Without an alias it’s difficult to see if we are selecting the correct alert when assigning to each monitoring process.

When a Slack notification for downtime comes through, I'd be happy to have it use @channel, to increase the chance that anyone on that channel will know about downtime as soon as possible. I don't see any way to configure that currently.

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)

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.

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 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.

Allow maintenance window for planned IT job (server down)