#1
I feel like ive learned my scales well, inside and out, modes and such, and ive learned maj/min/maj7/min7/dom7 barre chords, what other barre chords are important, i know i should know all of them, but what would you guys suggest i learn next? 6ths? 9ths?
#2
Whatever floats your boat, my friend. The two that you mentioned are probably a good bet, and I'd also recommend the sus2 and sus4 ones.
#3
Inversions of all the chords you know, plus the arpeggio's of them
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#4
Definately learn the aforementioned suspended chords - 9th's (whilst not usually 'barre chords') and 13th's are good - especially for more funky/jazzy stuff. I mean, learn as much as you can - it really depends on where you want to take your guitar playing. Learning different voicings of chords is always good for a bit of variation in songwriting etc.