To add a shelf under that significant, artful mirror frame, make sure your shelf holds its own under the mirror, so,ething like this (iron brackets and chunky wood) from Etsy:

Tried to post photo three times. Figured by now there'd be triple posts...

https://www.etsy.com/listing/601997319/shelf-brackets-steel-shelf-brackets-iron?ga_order=most_relevant&ga_search_type=all&ga_view_type=gallery&ga_search_query=iron+bracket+wood+shelves&ref=sc_gallery-1-2&plkey=5fd9bdcafda036001a8b1dc00f8b4cb0e15f7548%3A601997319&frs=1

Photo wouldn't load. But like above. Rustic.

Wealthy would be nice. :-)

In case photos don't load here are links too:

https://www.etsy.com/listing/638663926/small-accent-table-skinny-table-side?ga_order=most_relevant&ga_search_type=all&ga_view_type=gallery&ga_search_query=narrow+entry+table&ref=sr_gallery-1-12&organic_search_click=1&sca=1

Or a narrow (left to right) little entry table, which comes in many different stain colors

https://www.etsy.com/listing/273084668/console-table?ga_order=most_relevant&ga_search_type=all&ga_view_type=gallery&ga_search_query=narrow+entry+table&ref=sr_gallery-1-21&organic_search_click=1&frs=1&col=1

Like this but more polished maybe:

https://i.etsystatic.com/5902951/r/il/0950fd/1389347902/il_794xN.1389347902_ousm.jpg

https://www.etsy.com/listing/682161246/nightstandbedside-table-with-drawers?ga_order=most_relevant&ga_search_type=all&ga_view_type=gallery&ga_search_query=painted+bedside+table&ref=sr_gallery-1-10&organic_search_click=1&frs=1

I like where you're going. I would want the table or shelf to be a little wider than the mirror but not too wide as to seem disproportionate to each other. The depth of the table or shelf should not be deeper than the niche. I think a table that has storage or just open legged table is nicer than a shelf for filling the space. More useful too.