There have been some questions with users expressing their confusion about the fact that the bounty system gives you the option to "Reward existing answers" but you have to wait a full day, anyway (1,2). The need to award great answers seems to be there; I know I have seen posts that were not only great but also clearly required so much effort I wanted to reward the poster. But I could not, other than (ab)using the clunky bounty system. Voting is not always effective, especially on smaller sites and posts on niche topics.
Maybe it would make sense to move rewarding existing answers out of the bounty mechanic. Treat bounties the way they were intended, i.e. as incentive to post new answers, and give every answer (and question?) a tip jar where users can leave arbitrary (within some bounds?) amounts of reputation (with or without their name), effective immediately (maybe some tiny delay to allow timely undos).
Does this allow unchecked reputation transfers? Yes, but we already have a script that detects illicit/unwanted voting patterns, so I expect we can also detect reputation transfer abuse with high accuracy.
This would also allow for some new badges, for example "Tipped X rep to other users" and "Received X rep in tips" for several values of X.
Note how this would also give core-community members on sites where voting goes slow (especially small and young sites) the opportunity to reward productive users in a visible and noticeable way (beyond their one vote). This is a problem on some sites and hinders community kickstarting to some extent.