You can write your own SmartApps, but you have to host them yourself and then you communicate with smartthings through the REST API in the cloud so it’s a very different process than the old smartapps and I don’t know too many people who are doing it.
Sharptools has become a very popular option for those who want more complexity than the basic routines offer but don’t want to go into full on coding. It works with the new architecture. It doesn’t run local, but that neither did the old custom groovy smartapps.
There’s more discussion of various options in the following thread: