The base foundation stuff would come first, to ensure that Delphi 64 can be used for what Delphi 32 can't be used:

• compiler correctness: first and foremost, no internal errors, no incorrect code-gen
• ability to compile to 64bit DLLs and stability of those
• stress the memory manager: with large objects, fragmented allocation, multi-threaded allocations, etc.
• multi-threading: is it stable? is it efficient? does it scale? that for core RTL functions and units, and not forgetting the reference-counted types.
• floating point: does the compiler deliver proper SSE? are the maths functions properly implemented and correct? what happens if you stress the SSE register set with complex expressions?

And as a bonus, ability to accept 64bit object files from the usual C++ compilers.