For flexible indexing terms can be simple byte arrays, while the current TermAttribute only supports char. This is fine for plain text, but e.g NumericTokenStream should directly work on the byte array.
Also TermAttribute lacks of some interfaces that would make it simplier for users to work with them: Appendable and CharSequence
I propose to create a new interface "CharTermAttribute" with a clean new API that concentrates on CharSequence and Appendable.
The implementation class will simply support the old and new interface working on the same term buffer. DEFAULT_ATTRIBUTE_FACTORY will take care of this. So if somebody adds a TermAttribute, he will get an implementation class that can be also used as CharTermAttribute. As both attributes create the same impl instance both calls to addAttribute are equal. So a TokenFilter that adds CharTermAttribute to the source will work with the same instance as the Tokenizer that requested the (deprecated) TermAttribute.
To also support byte only terms like Collation or NumericField needs, a separate getter-only interface will be added, that returns a reusable BytesRef, e.g. BytesRefGetterAttribute. The default implementation class will also support this interface. For backwards compatibility with old self-made-TermAttribute implementations, the indexer will check with hasAttribute(), if the BytesRef getter interface is there and if not will wrap a old-style TermAttribute (a deprecated wrapper class will be provided): new BytesRefGetterAttributeWrapper(TermAttribute), that is used by the indexer then.