I apologize, I didn't realize those images are part of the digiKam website and not this one

Maybe the digiKam images should use Breeze instead of Fusion

I'm not sure I completely agree with this goal. While this may make sense for some simpler apps, I don't think KDE doesn't need to have apps for everything, especially when other open source alternatives have more features and their own dedicated community of users and developers. We would be reinventing the wheel just because "KDE needs more apps".
I have some specific examples in mind here, but I'm not sure they're all that relevant to this goal, so unless someone wants to see them I won't be putting them here.

Do you have some ideas for what "core software" would be here?

I think this goal should also include stuff about improving the existing user and technical documentation, like making it more complete, up to date, centralized, and easier to understand. The user documentation might also benefit from being in one place instead of either being in the Help Center, the UserBase wiki, the Community wiki, or its own website (like Krita and Kdenlive).
There is also still a lot of outdated technical documentation on TechBase that should be updated and moved to develop.kde.org. Most of the tutorials on there have messages saying that it's outdated and that there might be wrong information.

I thought about that when I wrote the comment but I wasn't sure how to make the comment not sound like that. I've edited my comment to just say "someone". I don't know if that's better, but "professional vs. amateur" isn't what I meant when I said "professional".

Would it be a good idea to have someone make sure the HIG is good and is actually used in KDE apps?