I’ve created a plugin that allows you to configure the New Blueprint dialog from Project Settings - If you are interested, please check out zomg’s editor addons - No C++ code required!

Manual setup

Using config files

Add into DefaultEditor.ini:

+NewAssetDefaultClasses=(ClassName="/Script/MyGame.SomeActorClass", AssetClass="/Script/Engine.Blueprint")

By default, the class name will be displayed, but can be customized using UCLASS(DisplayName...). Similarly, the class’ docblock comment will be displayed as the description in the menu.

 * This will show up as the class description
UCLASS(DisplayName="This will be shown as the name")
class MYGAME_API ASomeActorClass ...

(Thanks UKaosSpectrum on Unreal Source Discord)

Using C++

You can also customize the new blueprint dialog via C++ by registering a delegate with UUnrealEdOptions.

UUnrealEdOptions* Options = GUnrealEd->GEtUnrealEdOptions();
Options->OnGetNewASsetDefaultClasses().BindRaw(this, &UFoo::NewClasses);
const TArray<FClassPickerDefaults>& UFoo::NewClasses()
	return ...

This delegate is called every time the new blueprint dialog is opened, and the returned array is used to populate the list. Note that when filling in the FClassPickerDefaults values, the AssetClass must be /Script/Engine.Blueprint.