How to design connectors when I do not know what the customers want?
First, ask the customer what they want.  It's going to be a pig to wire up if you opt for one pin ordering, and their board has the pins in a completely different order.

If they don't know or don't care, then decide for yourself.  Pick a connector that's readily available.  If you're worried about connectors coming adrift, pick one that clicks in place.  And make it polarised, or someone will plug the cable in the wrong way.