The goal is to come up with a list of buttons so that I/we can have an organized layout that will work for whatever combination of toons we play with.

Please let me know what you think it a must have. The goal is to have as few buttons as possible that are as flexible as possible. For example, a general DPS button is probably vital. If I could set up all my DPS toons to have their main castsequence and then use that button to broadcast it to all, I now have a nice universal button so that I know if I'm playing all hunters, a mix of locks and rogues, etc, that I can spam that button and they will do their main DPS sequence.

Perhaps another example is a burst damage button. This is where I would put all my cooldown buttons for finishing someone off (maybe for a moonkin it would be moonfire while a warlock it would be shadow burn).

A poor example would be a force of nature (moonkin treant) button. This button would have no meaning to my mages or death knights. However, instead, I could have a summon temporary pet button, which would include moonkin's treants, mage's water elemental, etc. This button may not have any meaning to pallys since they don't have a temporary pet to summon, but at least I know that if anyone in my current team has a temporary pet to summon, when I hit that button the appropriate spell will be cast.

Which buttons can you think of that would help with a universal keyconfiguration so that when I load in any combination of my toons that I don't have to change my style much?

Summarized examples:
General DPS (broadcasts a button that maps to different castsequences for different DPS classes)
Burst DPS (for example a moonkins moonfire or a warlock's shadow burn)
Summon temporary pet (moonkin's treants or mage's water elemental)
....?

Please note that this is not a discussion of how to accomplish this using whatever key broadcasting method you use. ISBoxer had made it extremely easy to mix and match different toons and keep all the macros in order. This is purely just a discussion on how to keep your play style and key bindings consistent no matter which toons you are currently playing.