The barcode field should allow to hold further characters, for now probably only hyphens "-", vertical bars "|" and spaces " ".

The idea behind this is, that there are in fact releases (e.g. have a look at the 2CD versions here https://musicbrainz.org/release-group/783636e4-f89e-30eb-944f-ea0ca5ce0d9c) which are distinguishable only by the way the barcode is "printed" (not talking about the bars themselves, but rather the letters below), e.g.:

• 0 8122-75874-2 0
  vs.
• 0 81227 58742 0

Obviously the same barcodes (in terms of the number) but differently printed.

It would probably also make sense, to further allow multiple barcodes to be specified, but ONLY when these resolve to the very same number.
The idea here is that there may be SINGLE releases, which print their barcode in multiple fashions (e.g. the numbers from above are from one of the Superman releases, where one style is used on the back cover and the other on a cardboard slip-sleeve.
Of course we wouldn't want to allow having multiple different barcodes (in the sense of resolving to different numbers), as this would rather imply having another release.

In terms of API (XML and the like),.. the current fields should probably not change (and simply be generated by removing " ", "-" and "|" from what's stored in the DB.
There should however then be additional fields like "barcode_text" which give the written number(s).

What's the idea behind " ", "-" and "|"?
Well these are what's typically used for separating the numbers printed below the actual bar code.
"|" would simply mean that the numbers are separated here by the barcode itself being longer at that position.
I haven't seen it yet, but perhaps one should also allow = . _ / \ * and + as separators... seem like the usual suspects that may be used.