As you can tell from my avatar photo I'm almost 50, so I don't qualify as senior chronologically.
I do qualify from a medical point of view as rather aged, and not well aged. So I'll stick to that line.
The portable defibulator is a wonderful idea and putting it on the civilian market has opened the world to guys like me - watertight storage is a must.
Prescription medicine delivery services is something would be nice to see offered to boaters. Water tight storage built into the head for drugs is a must. Drugs must be in one central, easy to find place - and the head is what we would use at home.
Powered winches that don't cost a arm and a leg; I understand volume sales can drive the price down - but 5k for a 1.5k winch with a 1k windlass motor is really just socking it to the boaters. How many sailors go to the dark side of trawlers or quit altogether because they can no longer sheet a sail or raise the main?
Boat interiors that are more friendly to old knees - I think my Gemini is a good model for that, real steps/stairs at angles that are normal and 'intuitive'.
Berths listed as a sea berth should have manufactured (not jury rigged owner installed) straps and not require a leap to get in and get out of.
For a monohull - asking my step mother to use a handrail at 30 degrees heel discounts the fact she can't open a pickle jar, yet alone hold 30% of her weight in one hand.
Good luck with both your writing and your retirement. Hope I see you 'out there' for many years to come.