Is your feature request related to a problem? Please describe.
The main reason why I stick with Arial Narrow in my dungeon/raid WAs etc is because I want to make sure that fallback options are not uglier than Arial Narrow. If I were to use a font/texture that requires the installation of a popular AddOn and the user lacked them, it would fall back to uglier options like Fritz font etc.

Describe the solution you'd like
A fallback option that allows us to specify a font/bar texture if the user lacks the default selected option

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

I can't come up with a good way to enable this, I did consider:

a) Duplicating the statusbar texture settings so that you can configure a primary and fallback setting sounds like a lot of settings for that. Making everyone see all these settings is definately not worth, even if we are clever about the way we display that.
b) A hidden setting in the data, that is consulted if the primary setting is not available, but that also sounds rather too hacky.

If you do have a favorite progress bar texture/font and it has a compatible license, we can though simply include it in the addon.

There's a flip side to your request, which is the wish for "default settings", that is a way for users to configure a font (or also the default status bar texture) globally that is applied to every import. That has its own share of design questions and no one really working on it.