#1
I have noticed that there are multiple guides that can help people to improve themselves as a lead guitarist. A rarer thing however, were guides about how people could improve themselves as a rhythm guitarist or songwriter although these are basic skills that, being a guitarist, you should be able to have. So here, I made a list of the things that helped me improve my playing style.

Improve your chords:
I know that most famous songs consist of four to eight basic chords such as G C F G and I know that you don’t need a lot of (alternative) chords to write a good song. But, personally I think that, as a songwriter, you can distinguish yourself by using non-mainstream chord progressions. To me, if I write a song I try to challenge myself to put as much difficult chords as possible while it still sounds good. This is for me a kind of exercise to write more difficult songs so it keeps fun for me to play. Studying Jazz really helped me finding chords more interesting.

Improve your rhythm:
Every guitar player is useless unless he or she is tight on count. So before you start to use things like polyrhythm in your playing you should be able to play tight. Otherwise it just wont work. Practise with a metronome or drum samples. Let go the counting in your head thingy and learn to feel a straight 4/4, 3/4 and 6/8. Once you master this, you can start working with different time signatures or polyrhythm. But after all it is most important to play tight.

Study multiple styles of music:
By studying multiple styles of music you wider your horizon. Although you might not like al kinds of music such as Jazz or on the other side Hard-Rock or Metal, by studying and learning how to play these styles, you will discover unique techniques, chord progressions and dynamics which you can include in your own song writing and playing. For example, the first song I learned with a different kind of tuning was the song Slither by Velvet Revolver. The guitars were tuned in drop D (D, A, D, G, B, E). Now I use this tuning a lot in my song writing and not only for Rock. I also use it when I write a pop or folk song.

So all together, what helped me to improve my playing style and song writing was learning multiple chords and styles and learning to play tight. I hope that this also can help you.

Take care!
#2
I'd like to add to this because I think it's a very interesting concept. I've been in a lot of the bands, and I get a lot of people joining up as rhythm guitarists or bassists who believe that because they are not the lead player they can just show up with half a knowledge of the song and just sort of make it up, when in actual fact the people holding the rhythm are the most critical part of the band.

Changes
It's fairly easy to get a solid rhythm going on one chord, but the area when most amateur bands start to be less tight is in the changes, it is not just when but how you change that is important in remaining tight and it's important to know when something is offbeat and when it's on beat. It's also important to know the song to the point where you know what's coming next. You need to know if you play a riff 3 times, or 4 before it changes even if you don't understand the concept of bars. If you can't play a song without any backing then you don't know the song well enough to play it with a band in my opinion, which is why when learning songs for band practice I can talk my way through every song ie. 4 bars of the intro, 3 bars in the verse of riff 1 then one bar off riff 2, 4 bars for prechorus mad up for 2 bars riff 3 and 2 bars riff 4. Etc.

Using chords and tone
You mention using more interesting chords, but nowhere for me does that become more important than in bands with two guitarists. Rather than always playing the same thing, or playing the root power chords while they riff, if you get to a section where you are both playing chords, then you can really give it a different sound by adding a more interesting chord. No one will know it's there until you take it away, but it's important. The chorus to the song in my sig has a good example of that.

Also, using different tones can make it easier to hear what both guitarists are doing. It's not always best, because sometimes you want to blend together, but by using different amps, different guitars, different pickup selections or just different techniqes and eqing differently you can get a sound which allows you both to be heard without just getting in a volume war. Rather than turning up, try changing the sound. It's better for the entire band.

Just some thoughts anyway.