I have been with my partner for 16 years. We have dabbled in mild BDSM for the past year or so and I have developed a liking for it. My partner does enjoy it but I'm not sure she would want to take it further. Any ideas on how I can tell her I want to explore more?

Communication... Negotiate... Always play safe, sane, and consensual... Play with Safe words... Negotiate the do's and dont's,...and what avenues of play interests the both of you...
Fully agree...communication is key
I would begin with a pair of hand cuffs. Be spontaneous. These days, most of us are pretty liberated. Pull them out next time you and your partner are in a playful mood. Or, you could try posing with them on.
There are some very good steel toy cuffs with a safety release. That way, you can just begin with play. You do not need to begin with a lengthy negotiation.