No rolling the opposite direction of the gear. A car in drive can't roll backwards and a car in reverse can't roll forwards.
Whether this is accomplished with a half hold/creep mode or just a "The car always automatically 'Holds' at zero" (no special song and dance with the brake pedal) I'm fine with.
Auto hold at zero is probably the most straight forward. Basically, once the car is going zero only the accelerator can move it again.
Obviously this can be an option, though I would suggest the default for new cars is on, and if people want to recreate an odd bug-turned-feature from a manual car they used to own they can choose to do so.
This could also be "auto holds if zero and the brake pedal was used at all" if that needs to be the case for technical reasons, but "any zero" probably needs the least explanation (and would allow one pedal driving almost entirely).
I don't like that I have to have creep mode on, which has other ramifications, just to know I won't hit the car behind me.