What sort of limited feature set are we talking about? Anything that deals with palettes is a fair game, I guess since you can't do palette swaps/shifts with ARGB or even RGB images (other than doing graphical equivalent of search and replace I guess, but it's less performant and unneeded).
Also what visibility checks are you doing? If you are doing these to avoid overdraw, you can safely disable those I guess as modern devices are performant enough to handle overdraw in 2D space.
Perhaps if you'd write more about what algorithms stand in the way we could figure ways to change the algorithms involved so they'd work with 32bit images (other than palette shifts, since they'd be too cumbersome to do with 32bit images and should be limited to 8bit assets).