Currently, several elements in the WiX codebase have a Win64 attribute (Component, CustomAction, RegistrySearch are some examples). The Win64 attribute is not very intuitive, because its "yes" value is practically useless, its only purpose would seem to be to break your build when building 32-bit packages. The -arch switch (or creating your own custom similarly-purposed preprocessor variable) is a common way to fill out this confusing attribute automatically, which then allows you to rely on the default value to do the common thing, which you can then override with "no" when you want something to be 32-bit even in a 64-bit MSI.
I propose all Win64 attributes be removed and replaced with a 'Always32Bit' attribute, which will typically be omitted, but has two useful values - "no", the default, which follows the architecture of your package, and "yes", which specifies it should be 32-bit even on 64-bit packages. This attribute will not require the "-arch" switch to function properly.
I am very open to comments about the name of the new attribute.
This change will NOT remove the -arch switch, which is still useful for specifying the architecture of the package you want to output.
Wixcop migration of authoring will be quite simple for users who use -arch functionality for a sane default. Users who specify their own preprocessor variable to control bitness may be in for a slightly harder time.