What late model accessories do you wish your old car had?

Well they're not exactly accessories, or modern, but the intermittant wipers would be convenient.
I think that's about the only one if given to me would do in a heartbeat.
Maybe also a turn signal switch with lane change positions is convenient - car had one in it but it died.
Finally FM radio would be nice sometimes, but not enough for me to do anything about it.
To be fair, some of the items you've mentioned I've done:
Front disk brakes - factory option
Seat latches (not an option in 67 AFAIK) So modernized to 1970
Headrests. Done.
H4 Headlights & relayed power.

Oh there was one other thing. Cloth seat centers. :)