Referring to KDE, but most likely other DEs work the same way. Since many installations have multi-user setups, Manjaro misses a better user management at the moment. Here are specific missing features and their benefits:
Option to allow standard user to change his password. In short, it's a huge security weakness. Searched the forum and found similar questions earlier, and a few users suggested that system is more secure if only super admin can change user's password. Actually, it's the opposite, and here's a recent (last week's) story - a friend of mine has Manjaro installed on his office (a small local company) and he is super admin, while office workers are standard users. One of users suspected his password became known to a colleague, so he wanted to change it. Obviously, Manjaro didn't allow it, so he called to admin asking for help, but he was out of country and couldn't assist. So user had keep his old (possibly leaked) password. A few days later (office is monitored by cams) one of colleagues logged in to his account and performed some stuff (no need for more details here). As you see from real life example, it's a terrible decision to prevent users from changing their own passwords.
If option #1 is implemented, we can also implement an option to force users changing passwords every X days/weeks/months for better security. It's available in many other distros, and such option would add the same security benefits to Manjaro as well.
Single, but more powerful user management tool. Manjaro KDE has 2 tools to manage users. Both have the same limited features and very similar look. In short, they duplicate each other, and I see no logical reason why 2 duplicated tools are included by default. Instead, we could have only one, but a more powerful tools which enables not only to add/edit/delete users, but also change their permissions the easy way. For example, Ubuntu based (and some other) distros allow to enable/disable individual features (USb storage mounting, etc.) in user management window. Manjaro has a similar feature, but it's much more complicated because it only allows to assign users to specific groups (like "optical", "lp", etc.) but these are not clear to 99% of users.
Hope I explained all the features clearly enough. If not, please let me know. Waiting for other users' feedback and comments.