There are two features which are useful in Taerget Classic, but no in Target Standard. There two features needs to be implenmented for smooth migration.
In target standard you can use response tokens to achieve the same use cases as you would do with Plugins. Response tokens are similar to plug-ins: they let you surface Target-specific information to use elsewhere. Response tokens let you choose which variables to leverage and then enable them to be sent as part of an mbox response.
More information on the same can be found here Response Tokens
