Starcraft 2 has simplified many duplicative work compared to Starcraft 1, for example, players can now form multiple buildings into columns. However, casting is still trivial. To control a caster, players either use [tab] or control multiple groups of units at the same time.
This inspires the idea of enabling all magic casting when players select a group of units. For example, you select marines and ghosts then you can cast stimpack and emp without [tab]. I believe this can boost game experience of all three races. This has already been applied in some lobby games and only need a small change of UI.
Some may be concerned about the balance, but new patches can probably handle this problem.