SUMMARY Now that font, icon theme, cursor and layout settings of GTK apps follow Plasma/KDE/Qt preferences, the only options left in the GTK Application Style page are the GTK2 and GTK3 themes. I suggest moving these to Application Style page to reduce confusion. The 2 dropdowns could be displayed at the bottom of the Application Style page (after the scrollable Qt style selection grid), or on a second tab. I propose behavioral changes as well: selecting a theme from either list that has both GTK3 and GTK2 variants select the same theme in both dropdowns, for the sake of consistency. This way (I believe) customizing KDE would be a bit easier. I didn't include steps to reproduce and system info as it would be irrelevant in a design-related issue/feature request.
Yep, this is the plan! See https://phabricator.kde.org/T10611 No need to have two places tracking the same thing; let's keep T10611.