What if the special "modifier" squares, like sentry or riot shield, were usable as normal squares (and you can have up to 8), where you have to replace a color with it, but you can put it anywhere in your song. So, if you want a sentry, you can do Yellow, Sentry, Blue, Purple and have a sentry that does what it currently does but without a green. Perhaps they could be colorlocked as well, e.g. in the above example, that'd have to be a green sentry.