Package Management – How to Add the ‘Proposed’ Repository

There is no list that covers all yet, especially for possible future updates/upgrades. Maybe somebody can work on this ...

That is what I could find out so far: If a program is updated/upgraded, it will be automatically added to the software centre (and will be updated by the system), viewable also with Synaptic (needs install) and apt-cache (command line). The page packages.ubuntu.com shows past updates, but do not inform what the future will hold.

Stable Release Update/Backport
Every SRU (Stable Release Update) and Backport is manually requested. SRUs are done only to fix an important bug. Backports are done upon request, if volunteers and resources are available. Requests for packaging and/or backports are generally tracked as bugs. They are not really bugs, of course. But bug trackers usually track more than just bugs, see backports list. If you poke around in Launchpad, you should quickly find the similar tracking lists for the Packaging Team, the Security Team, and the SRU Team. There is doubt that tentative release dates are realistic for most packages, since many backporters seem to work more-or-less at random from the list. If some backporter has specific interests, then they could certainly post ETA dates in the comments and assign the package to themself.

PPAs and alternative repositories
As there are not many significant updates/upgrades that are added in the official repositories of a Ubuntu release (although this might change with future LTS releases), PPAs and alternative repositories are chosen by a lot of people, but it takes time to find them:
I would also promote a list of PPAs and alternative repositories, but this is not supported by Canonical as many PPAs and alternative repositories can cause problems to a system. But a list with the possibility of rating and comments (problem, success) would be better as the scattered information without or little feedback from the community. Unfortunately the Ubuntu brainstorm page will be closed, so I add this idea to this answer, perhaps somebody knowledgeable reads it and can work on it. There is a related brainstorm idea regarding this topic as long as Ubuntu Brainstorm exists.

Check that the only file in the below folder is multiarch using:-

ls /etc/dpkg/dpkg.cfg.d/

See if the output is multiarch. If the output is multiarch then run below:-

sudo sh -c "echo 'foreign-architecture i386' > /etc/dpkg/dpkg.cfg.d/multiarch"

NOTE:- First, check the output of uname -m, if it is already i386, then no-need to follow above steps, i.e, i386 is enabled by default.