Cthulu is great if you are looking for Bach Chorales (Chords based from church hymns on Sunday mass)
If you are looking for something free though, check the plug-ins at CodeFN42!
Can't post any links at the moment unfortunately but google: CodeFN42 and their site should pop up
Chordz and Cales really help a lot if you're looking for easy access to any chord scales and melodic scales to go along
Cales allows your midi to begin at any melodic scale from any root note starting the first note on your piano roll to the note of C (Chordz does have Cales built into the plug-in as well!)
Chordz allows your midi to begin any chord scale from any root note starting the first chord on your piano roll to note of C
RandArp is amazing as well if you are looking for an simple arp plugin (quite similar to Ableton's Arpeggiator) and you can even link Chordz to RandArp to arpeggiate your chords
These are really helpful plugins for learning your chord structures or if you are looking for easy accessibility to chord structuring, all 3 plug-ins have helped me out a lot!
Hope this helps everyone out here and if you do want a pdf to learn more about music theory on a simpler terms in writing, let me know and I'll post a link to the pdf on here!