Still enjoying our retirement. However a thought creeps in to my head, "What next"?
It's going to depend on your needs as you age, and your needs will change. If you want to avoid the nursing home (at least for as long as possible) then look to adapt your home while you still have the capability to do so. As others have noted, you will reach a threshold where it is no longer possible to realistically change stuff. Also, if you change it now, you will have a lot of time to get used to it and not have to adapt to it during or after a crisis.
Aim for at least a downstairs room that will be the designated bedroom; maybe even kit it out as such. Install a downstairs accessible toilet and shower. If you can make the entrance of your house accessible (if it's not already) that would be brilliant.
In houses where this is not possible, the obvious answer is a stairlift, which is fine, but not something you will want in your house until you absolutely need it, which means change during or shortly after a crisis.
Care homes become necessary when your needs escalate to the point where you can't realistically be on your own without getting into difficulty with everyday things. You can stay in your own home with additional care (meals, care visits, overnight carers) but sometimes the care needs just become too much to supply outside a dedicated nursing home based setting.
That said, I've seen it said here before than only 1 in 20 of us will end our lives via a nursing home.