The best I can do is make a rather big map with a city (which would take AGES so Im not sure if I want to start such project) and add some random objectives where you try to save survivors and stuff like that, with trigger spawns for specimens so they attack you only in certain areas (maybe excluding a few odd stragglers here and there). But it wouldnt be very amuzing for long until someone gave you an option to save your character/items and all that :/
I think that making a map large enough to be considered big compared to the medium sized current ones, would not be as hard as you think, as long as you plan and think first, before starting, there are cons but also pros.
Some of the pros is that you would not have to worry about camping, because the trader being really far away enough would force players to move considerably, not talking about only straight line distance, but how complex it would be to reach it, using elements like waiting for Elevators, coded Doors like The Hive one and etc.
Personnaly i HATE all maps available for this game, that are entirely planned to have zero camping areas, they become very artificial and unrealistic due that, they have zero 'interface' with the player, maps are 90% made of looks, the 10% left is made of weldable doors (even these respawn after each wave), zero features, zero variables, the Hive at least have elevators and trains and coded doors etc, even thought its a mission map.
For now, there still not much done to change KF, i would put my money at some easier but allready done things to change the gameplay:
1 - A planned large map, using the keys i mentioned, like far away traders, some elevators, coded doors (if not too much trouble), maybe trains passing by in a tunnel (like the cars in the map Gas-Station) forcing players to risk passing to other side, and maybe even switches that activates something like a trap used in same gas station map car washer.
3 - Map should require much LESS testing since you would not have to worry much about camping and spawns locations etc, even less if using benjamins Ai mutator, because you could make very few spawns and yet specimens would fill the whole map.
2 - Wait for Benjamin's Ai mutator, that would make everything a thousand times more interesting, and should allow for plenty more options and type of gameplay.