Recommended replacement for Field Definition File (TTX) based reports?

We have about 90 legacy reports based on old TTX (Field Definition Files) that are built in source code. It was mentioned that these types of files are deprecated and are not going to be supported in the next runtime release.

What is the proper way to design and populate reports based on variable data from source code? DataTables seem to be out, as is TTX. We'd like to start moving to the new proper format. Thanks!

Kyle