Hello! Thank you for the kind words! :) You can already replace colors with the bucket tool by selecting the "Same color pixels" fill area in the tool options. This will replace all of the pixels of the current cel of the same color, with another color.
As for your other requests, we would like to work on them at some point for a future update, but they're not in our current plans yet. Right now we are focusing on remaking the selection system for v0.9!