Hey Friend,
Normally I'd have an exercise for you with chord changes between the I chord and the one we're currently studying (the IV version of A major).
Alas, I haven't made the exercise files yet, so I wanted to talk a little about something I just learned (and you can use right away, too).
As we've studied the chords of the various keys, I've given you the "stock" harmonic scale or progression. Each chord you've seen is made up of notes from the scale of that key. For example, in the key of E major, the scale is:
1 2 3 4 5 6 7 E F# G# A B C# D#
The triads we've been working with are:
I IIm IIIm IV V VIm VII° E F#m G#m A B C#m D#°
When you listen to these chords in succession, they advance up in pitch very harmoniously, as you would expect. Each note of each triad advances up in the proper intervals to give a pleasing scale-like sound.
From these "standard" triads (chords), you can play a gazillion Rock songs without ever needing to step outside these basic shapes.
But what if you want to through the listener for a loop with something that doesn't quite fit? Well, you can without damaging the integrity of the key.
What's That Sound?
They're called "altered chords." In effect, all you're doing is strategically changing one chord note by a half step (one fret). This alters the character of the chord, and you'll likely only do this with one chord in your progression.
In most cases it will be the 3 note of the particular chord, but not always. You could modify the 5 note as well (making the chord diminished if you lower the 5 note by one fret, or augmented by raising the 5 note by a half step).
According to Tyler Larson, the chord where you'll have to most success and dramatic effect IS the IV chord, which we just happen to be studying right now! A major is the IV chord in the key of E major. Here's an article he did for Guitar World Magazine where he explains it.
The easiest way to spice up an otherwise drab Rock progression is to change the IV chord to a IVm (minor IV). Take a watch of the video to hear some examples. As you listen, think about where you might have heard these progressions before.
We'll start looking at this kind of stuff in more depth as we go along. Gotta run, but I'll be back tomorrow with more fun!
Peace~
Dave | 
