Is your feature request related to a problem? Please describe.
There is currently no option to load based on GetSubZoneText(), only GetRealZoneText(). (And Zone IDs of course.)

Describe the solution you'd like
Either an additional option in the load conditions that works exactly like the existing Zone Name condition, just with subzones instead.
or
Add support for subzone names in the already existing Zone Name condition, similar to how the Zone ID supports both individual and Group Zone IDs.

Describe alternatives you've considered
You can check for the current subzone using custom triggers, which is effective and makes this feature request not strictly necessary. Adding it to the load conditions seems relatively straightforward however, unless I'm missing something, and might be a nice small quality upgrade in some situations.

What's the use case for this?

You'd use this if you want a weakaura to only load in a specific subzone rather than in the whole zone.

For me specifically, this came up when I was trying to make a reminder weakaura to change my talents/equipment in a raid. Different bosses in a raid are often in different subzones, so if I want a weakaua to remind me to swap talents for a specific boss, loading a weakaura based on subzone could be helpful.

This could also be useful if you only want specific weakauras to appear in a specific part of a city. Perhaps you have crafting related weakauras that you'd only want to load while in The Artisan's Market, rather than in the whole of Valdrakken. A load option would allow you to do this without custom triggers.

There's no locale-independent way to check for the sub zone, as they apparently have no id in the api. I'd be very reluctant to add anything that will lead to locale-dependant auras, it needs to come with some much needed benefit.

The boss specific reminder, while somewhat useful, doesn't really clear that bar. And additionally the sub zone to boss mapping is not unique in pratice nor in theory, so for that it would be a crutch.

And there are addons such as LoadoutReminder, which are exactly for that purpose.