This can actually be quite minimal. You need a name. You can do this easily via a Suitelet.
Of course if you are going to turn the customer record into an actual customer down the road you need to set enough information that you can retrieve the record otherwise creating it in the first place is pointless.
So in addition to the name you need to think how would you link the placeholder to a real person who is willing to offer some identity.
A possible value add is to include some information that identifies the original customer creator or an associated product so you can track revenue or get some sense of campaign success if the placeholder becomes a customer.
If you are doing this work as a contractor you should of course find out from your customer what there minimal requirements are. Perhaps you should create the customer record as a prospect or a lead instead of as a customer.