Add support for a SHARD KEY in the table definition. This is passed down into the storage engine as a hint. The storage engine is free to ignore the hint.
Forward port all query optimizer enhancements implemented in SQLite3 over the previous year into SQLite4.
Review all other changes made to SQLite over the previous year and forward port all that seem appropriate.
Use decimal arithmetic everywhere within the code. Avoid the use of IEEE 754 binary floating point, except for APIs that have "double" parameters or return values.
Carefully review the entire API and look for other ways to make it simpler and more robust.
Change memory allocation routines to be methods of sqlite4_mm instead of methods of sqlite4_env.
Add a new extended error code for every error where doing so makes any sense at all.