It should be easier to add custom fields since it is a common practice to build custom content elements and these content elements my add fields to tt_content.
A possible implementation could be to add a field in the indexer configuration record where it is possible to add custom fields (like currently "Content element types which should be indexed").
Currently it is possible to index custom fields added to tt_content by adding two hooks, see https://github.com/teaminmedias-pluswerk/ke_search/blob/master/Documentation/Indexing/IndexerTypes/CustomContentFields.rst
It should be easier to add custom fields since it is a common practice to build custom content elements and these content elements my add fields to tt_content.
A possible implementation could be to add a field in the indexer configuration record where it is possible to add custom fields (like currently "Content element types which should be indexed").