It’s actually “cold air return” because it’s returning conditioned and cooled down air to the furnace for recirculation and “combustion air” that supports burning process. The reason these two supposed to be separated by at least 10’ (which distance requirement applies if they are located in the same enclosed area and it applies to any combustion chamber i.e. furnace cold air return – water heater’s combustion chamber) is to prevent bi-products of combustion process from being re-circulated through the house, in other words; sucked into the air return port.
For example; if you have malfunctioning burner which results in incomplete combustion, or if there’s a vent pipe / chimney problem (i.e. obstruction in a chimney), which would cause Carbon Monoxide spillage into the crawlspace, that Carbon Monoxide gas would be immediately sucked into the furnace’s cold air return port and carried with air through the registers in your home.
With direct vent appliances the combustion air is being supplied directly to a sealed combustion chamber from exterior so there’s no immediate danger of contaminating home’s conditioned air - cold air return can be located right next to the combustion chamber because it is sealed.
Open combustion chamber appliances require oxygen to support burning process from the appliance surrounding area, which of course can be supported by additional exterior ventilation ports, and it is always supported by air exchanges provided by doors, windows, and other “holes” in building’s envelope.
Additional fresh air supplying port in the crawlspace’s wall will not change much in your situation because the combustion chamber and the cold air return port are still next to each other. It looks like your only solution is to extend the furnace’s air return through the floor or wall.
Maybe you should ask your local HVAC contractor for some advice. I don’t know the configuration of your heating system and the floor plan of your home so I can’t really suggest any particular (the easiest) way to do it. Whatever you do, make sure that you have functional and properly installed smoke alarms and Carbon monoxide alarms.