I love this idea so much. What a lovely thing to do as part of making your relationship a more formal agreement. However, I have a feeling that you are a bit like me and the moment someone has put you on the spot to come up with something unique and special, your mind instantly goes blank or you begin to completely overthink it.
If you have some ideas but it is making a final decision on one that is the problem, then tell your Dom this and ask him if you could possibly present him with a short list of say 6 suggestions where you will explain why you have picked them. Then, he can make the final choice.
However, if the problem is that you just cannot think of anything at all, here are some tips to help you.
First, it should be a world that you would never usually say in those circumstances. So, things like 'No, Stop, Don't' are best avoided as are nicknames, terms of endearment, and expletives. This is because all of those words could potentially be part of a scene and then misunderstood. For this reason, I would also avoid words that rhyme with words such as this that could also easily be misheard. For example, fur rhymes with Sir, dove with love, luck with both fuck and such, etc.
Second, it needs to be something that you will both remember. So, a word that has no discernible meaning to you or that is too abstract for you to remember is best avoided. It is very possible that the moment when you need to be able to remember it will be the moment when you body and mind are under pressure. If it's not a word that you can easily recall because of its greater meaning or a memory associated with it, then you are more likely to have trouble remembering it when you most need to.
My safeword, which I tend not to tell people as it is something very personal between my Dom and I, is a word that means something to both of us. There is a story attached to it which comes from the early days of our online communication. This not only made it memorable but also made it special to us. You could just grab the dictionary and let it fall open on some random page and then blindly pick a word that way, but it is unlikely to end up feeling like a word that is personal and special to your relationship that way.
At the end of the day, you must pick a word that works for you, that you feel comfortable with, and confident that you will remember it in those moments when even remembering your name feels like a challenge. Good luck and congratulations on your collaring.