If you want to make a new trait it should not double the others. A food trait would be something like +1 food with all health building(granary, aqueduct, grocer, hospital and supermarket), so it would be a long term trait. Seafaring can have another name but there is no traits what is tied with sea. Vikings would be seafaring/agressive, so their ub would give free navigation 2 when trait gives nav1. Also +50% ship production. Air trait would be useless because first air unit is airship and it comes littlebit late.