Unboxed structs for Java
The cost of allocating and garbage-collecting heap objects is usually justified - but it is none-the-less often non-trivial. Furthermore, the concern about the cost (whether justified or not) causes programmers to write more convoluted code and design more complicated APIs. Some object-oriented languages (most notably C++ and C#) provide for struct types that may be stack-allocated and optimized in various ways. There has long been interest in such a mechanism for Java; here are some thoughts.
Co-ordinates, rectangles, and affine transforms in graphics programming.
Arbitrary-precision integer, as a pair of a 32-bit plus a pointer to a extenstions array. (The latter is non-null only when it is needed.)
Tagged immediate object types (as in unboxed fixnums), by using a pair of a int (or long) combined with a pointer.
A Swing Segment.
Document position and nodes (as in Swing or XML).
A value type must be 'final'.
Mutable or immutable?
Equality and identity.
Boxing of structs.
Arrays of structs.
Backwards compatibility: Can we design the feature so it is "optional", in that older or more limited VMs can correctly execute bytecodes, that will be efficiently executed on supporting VMs?