Actually, Windows Vista
is based on Windows Server 2003, and has been since the end of Milestone 7 (Build 4093).
After the failure of the Longhorn development cycle between 2001 (according to anti-trust documents) and 2004, Microsoft forked the 'post-reset' Windows Vista from a build of Windows Server 2003. In fact, the first 'leaked' build of Longhorn 'post-reset' in 2004, is
Build 3790.1232, which is basically just a build of Windows Server 2K3. Also, Windows XP and Windows Server 2003 are not 'two distinct' kernels. Windows Server 2003's kernel (NT 5.2) is fundamentally Windows XP with a few tweaks that were later backported to XP in SP2 and SP3. Windows 7 is similar to this; it's basically Vista with a few tweaks to make it more efficient. 8, 8.1 and 10 share this as well.
If you look at Windows Vista build 5048 (WinHEC 2005 preview build), it is based on the Windows Server 2003 codebase. It is only a brief hop to Windows Vista's release build of 6.0.6000.
So
@kasumigenx, your entire premise is already fulfilled by OTL.
And Windows XP for 64-bit computers already exists. It existed for Itanium 64 (Intel's first attempt at 64-bit computers before AMD innovated x86_64) and Windows XP x86_64 (after the release of the Athlon 64). Neither one caught on because almost nobody had 64-bit processors in the early 2000s.