I have spent quite a bit of time today thinking about your original question and have now read your later reply. I don't really know why your question has so stuck in my head but on re reading it and the later comment, I wonder if props are perhaps only part of what you might find helpful
to make the images a bit more interesting
I guess you are photographing adults for at least some of your work
Seems that people are more than happy to let me practice on them
so you may have the chance to do more interesting stuff than with children.
I am speculating that perhaps you are finding you have 'been there, done that" a bit for portraits and are reaching for props to sort this (which is both good and fun) but maybe you also could consider reaching for more dramatic light or a change of poses? If that interests you at all, then google images is your friend.
I would recommend searching under "hollywood portraits 1920s" and search through the decades 30s, 40s until about the 60s - the quality of the images plummeted in my personal view after that, so its less easy to learn useful stuff. The earlier decades stuff is really the most imaginative. Often the black and white images show the structure of the picture most clearly, as its possible to get distracted by colour a bit. Its easier to see what they did and how they did it.
Maybe look at the images you like and think about
- the textures of everything in the image
- the props
- what is in the background, what the background is made of (draped fabrics, staircases)
- the poses
- how the light falls on the person and the other things in the shot, include the background itself
- is anything interesting being done with shadows (many shadows are intentionally cast on faces or bodies by placing things between the lighting and the subject)
At the top of this page are 4 very different portrait looks
https://sistercelluloid.com/2015/11/05/world-war-i/
This page is full of textures
http://monovisions.com/portraits-of-hollywood-actresses-1930s-vintage-black-white/
This is the same woman showing many aspects of her
http://www.vintag.es/2016/04/30-stunning-black-and-white-portraits.html
All of the above rely lots on shadow. Lots of modern images fear shadow, particularly with women who tend to be lit in a bland and flat way. This does indeed flatter if you want to hide every flaw and line, but it also removes facial structure and looks pretty boring. Your eye is directed to nothing and wanders about trying to find a feature.
Do not be afraid to tidy up bits and pieces in photoshop or lightroom, the old time photographers had very skilled retouchers who did most of the work on the negative with pencils, little room for error there really! This is a very well known image of Joan Crawford before and after retouching
http://rafaelalexander.com/wp-content/uploads/Screen-Shot-2014-01-30-at-2.29.07-PM.png
(EDIT - just to clarify, I do mean tidying up, not turning the skin to plastic - the person who does Damian Lovegroves retouching for him does a fine job of keeping skin looking normal while taking out short term blemishes etc)
It might be worth taking a look at the old time / golden age hollywood photographers and in addition maybe the work of
George Hoyningen-Huene
Horst P Horst
Norman Parkinson
Damien Lovegrove (modern photographer)
The work of these photographers may or may not be to your personal taste or aspirations, but it is much more daring and exciting than much modern stuff. Perhaps you can find ways to modernise or adapt some aspects to your own taste and interests.
Maybe put a radio, some favourite music of your volunteers on or an audio book to keep your models from getting bored (no visual stuff as they will move their heads to look at it) as it gives you a bit longer to work on the forming of an image. Subjects do not need to smile as many beautiful images are made without smiles, so they don't need a fixed expression and can just drift off a bit mentally while you set up and play around with your test shots, props and lighting
I hope this helps in addition to the prop stuff. If I am mistaken about your reaching out for new ideas, just disregard this post 
All the best.
