I’m not sure if you are familiar with 3ds max, but there is very useful “mini dope sheet” like timeline. There you can visualize keyframes with different colors (pos RED, rot GREEN and scl BLUE for example) and you can slide and reposition timings. You can also select multiple keys and scale with range and so.
Most of time I don’t use dope sheet at all because I can do all simple retiming stuffs via timeline. There is also nice filtering function to see only active tool (pos, rot, scl, materials and such) which can also speed up your work.
Many good things there, but not only good. Even there is filtering system, it’s not so fast to use it since you have to right click and select from list. For example if you have pos and rot keys at same frame, there is no way to separate them without opening curve editor/dope sheet or set timeline filter to show only pos or rot.
So maybe one solution for good “timeline dope sheet” is to have low profile keyframe rows with display filters. By default all shows together with single row (you can still see different keys with different colors). If you want to move rot keys separately, just click expand button from timeline label and you will see pos, rot and scl rows separately or depending how your settings are. If there is multiple selections, expand won’t show all object separately, but all position keys are on pos row and rot keys are on rot row and such. So you have option to handle all key frames at once or different properties separately. If those are not enough, then you really should open full dope sheet! 
After good display system, we definitely need some move (slide), delete, scale and copy/paste or slice clone functions to speed up our work! And when you are sliding or scaling, you really want to know from and where and how many frames you are sliding or how many percent you are scaling!!! Small info box when handling key frames would be very useful, don’t you think?
What about play animation range? Usually I want play just part of my animation to check what I have done and not whole scene. Quick animation play range would be nice. might be good idea to have play range and play whole timeline buttons separately?
EDIT: Oh, there is already preview range, but it works only once and it will forget range.
OK, those are my first thoughts for now. It’s very fun to through some thoughts and discuss what might work and not. But hay, now I have to get sleep 