If you'd like to implement this type of feature, I'd really consider using a /Click Castsequence builder within your application.

Have it time how quickly you spam your key. Based on your spam speed, have the "wizard" place the correct number of comma's.

Choose spell 1, spell 2, spell 3 etc. You could go with a graph for each individual sequence. So people can see the cast time for a given spell is 1.75 seconds. The spell duration is as long as the red (pick a color) bar goes.

Then have the ability to put in multiple sequences, and prioritize which is more important.

That way, you're offering something that makes Click Castsequences much easier. At the same time, you're not doing anything at all, which could not be accomplished within the basic wow ui, without any macro's at all. Cannot really get into too much trouble, limiting yourself to what can already be accomplished. But, offering it in a way that's a ton easier to create the macro's - Just pick the spells you want and have the wizard build the macro's for you.