Look for applications that still use KService/KPluginInfo/KServiceTypeTrader/KMimeTypeTrader (i.e. desktop files in ksycoca) for plugins.
Any use for application listing is fine and can be ported to KApplicationTrader - D25698.
Loading plugins for an application can be done with KPluginLoader/KPluginMetaData (kcoreaddons).
Loading KParts can be done with KParts::PartLoader.