Right now, you can store selections in selection masks, and reactivate them.
You can also select the global selection mask and paint on it, as well as view selections alternatively.
Here's what can be done better:
# [DONE] SelectionMask Painting in Realtime. - https://bugs.kde.org/show_bug.cgi?id=343217 - This is a fairly important one. Right now, no matter which view mode you use, you cannot see the mask updating while painting. The old brush-selection tool had a canvas decoration it painted, but none of the current tools allow this.
# [DONE] Multiple 'local selections' are allowed but not possible. - https://bugs.kde.org/show_bug.cgi?id=341973 - This is actually due to the way selections behave right now. A new local selection becomes automatically activated, which means that every selection change you make afterwards edits this local selection. A user needs to deactivate the selection to start a new one, which might be a bit of a workflow issue?
# [DONE] Add opaque, intersect opaque and remove opaque from the global selection - https://bugs.kde.org/show_bug.cgi?id=346892 - This would make selection masks really really powerful.
# [DONE] Create selection when enabling Show Global Selection - https://bugs.kde.org/show_bug.cgi?id=343218 - I tried adding this myself, but it seems there's a million checks everywhere to deselect a selection when it's empty.
# [DONE] Add the ability to convert vector selection to pixel selection, and the ability to transform pixel selection - https://bugs.kde.org/show_bug.cgi?id=359421
# [DONE] Allow modifying selection - https://bugs.kde.org/show_bug.cgi?id=369341
These both focus on how it's not obvious how to transform a selection. This might be partially because the global selection masks isn't visible by default?
[Avalable for vector selections] Right now, Krita doesn't use the vector boolean stuff anywhere, but we will be implementing it for the vector tools. It would be cool if we could also implement it for vector selections.
# [DONE] When using Selection Tools, make right-click to bring up the content in Menu>Select - https://bugs.kde.org/show_bug.cgi?id=337380 - This basically asks for a pop-up palette specially for the selection tools. Could be interesting.
# Let Mirror/Rotate Layer work with a selection when it is present and assign shortcuts - https://bugs.kde.org/show_bug.cgi?id=365595 - Basically, use the selection as a boundingbox for the layer transform actions.
# Unable to see marching ants (selection) with grid enabled - https://bugs.kde.org/show_bug.cgi?id=362662 - This is a drawing order bug.
# [DONE] Rounded Corners for Rectangle Selection Tool. - https://bugs.kde.org/show_bug.cgi?id=335568 - This is more of a geometry-tool suggestion.
# [DONE] Selection with Smooth interpolation. - https://bugs.kde.org/show_bug.cgi?id=351787 - As stated in the triaging comment, this would basically be a new tool.
# [DONE] Implement selection mode cursors with David's cursors: bdc4db9abea7b6ce8be97c5633ba802a9887ef1e - https://bugs.kde.org/show_bug.cgi?id=385861
# Lazy Selection - T3493 - Use the colorize algorithms and friends for foreground selection.
# Magnetic Lasso Tool - T556- A smart-selection tool.