To answer the original question asked:
How big do you want it? - Small and slim
Do you want other ports to be able to charge other things with it? And if so, what kind of ports? - USB 3.0 or C, not a major need though.
Do you want data transfer through the charger? - Would be nice but not essential
Should it plug directly into the wall or should there be cable? - Directly
Should the wall plug be replaceable by the user? - Yes
Should it only include one localised wall plug, or should you get them all included? - UK, EU, US included
Should Eve put their logo on the charger? - Yes
Do you have any stand-out examples of chargers that either did it right or did it wrong? - See previous comments. Surface pro 4 charger brick size is a good dimension and weight.