This article will not be about Steely Dan, I only use them as a example what you can do with your solos.
Steely Dan were well known for hiring different guitarplayers to create different sounds into their songs. Each guitarplayer they hired had a unique take on playing solos. At some point even Mark Knopfler of Dire Straits was asked to contribute some of his single string skills to one of the Dan’s songs.

In an ideal world guitarplayers should have enough theorectical and technical knowledge to execute exhiting solos for whatever song they are asked to play over. Because the world is far from ideal I will give you an insight how you can approach the subject of playing great solos:

      
      Mood of the Song and Your Solo:

Whatever song you listen to, most of them will fit into a certain style, a particular category of music. Usually this style will draw on a set of rules, sounds if you like, what creates the unique sound of the style. This style will have its own rules on what kind of solos will work, they are the so called stylistic cliches. Listening to particular styles a music will give you an insight in the sound of this kind of solos. Hopefully, over time, you will learn to play solos using some of these stylistic cliches. They will not make you sound great or unique but they will work and will teach you what to play and what not.
Once you know how to work in some styles you may want to play about with the cliches and use them at your own disgression. Later more on style.

When it comes to style, Steely Dan were unique in joking about with the style, they created solos in pop song which used a lot of jargon from Jazz Rock, or whatever genre they were thinking of at the time, to make them fit the right mood of the song.
This kind of approach makes up for more creative- and individual solos.

When it comes to mood each song conveys a particular mood, this mood can often be defined in day to day language instead of musical jargon. A song can sound angry, happy, sad, upbeat, shy ect.
You may want to get your solo fit the same kind of mood of the song, this would be logical. On the other hand you could also go in opposites: Create an upbeat solo in a sad song.
Is this a hard thing to do? Well it is just a matter of trying. Most theory about scales and harmony will try to teach you just that, but you could also get similar results by thinking about your playing and experimenting. This may be more fun. True, you have to approach your guitarplaying in different ways each time you play, and you will only get there by hard work (it is all about playing really!!)
Creating a contrasting solo with regards to the mood of the song will do a similar thing as what the Dan tried to achieve with their songs: All these different guitar greats had different takes on styles and the sound of the guitar. All in all you need to become a cameleon with a lot of different colours, but it certainly is a lot of fun!.

          Harmony versus Melody:

To get different moods in your solos you could think in terms of harmony or melody.
Playing solos where you use only chordtones of the song will get you to play harmonically. It will give you different results compared to playing melodically.
Melody, on the other hand, may disregard the chords of the song and live a life on its own.
Successful melodic solos can stand on their own without support of any chord. Harmonic solos may find it hard to make sense on their own.
Ideally you should be able to play in both ways. Just think of how you approach your solos and start to experiment with both ways of playing. Sing along with whatever you play, this is always a great way to improve your melodic twists.

         Style:

If you are familiar with particular styles of guitarplaying why not experiment with any of the following: Play bluesy over a non-blues song.
Play atonal with lots of dissonant (wrong) notes, and slip back from time to time to playing in the right key of the song.
If you are mainly a pentatonic guitar player why not slip in some chromatic notes to spice up your sound. Disregard your pentatonic patterns and just play some notes where you go up fret by fret over one string. Create some fast runs in this fashion to get some energy in your solos and before you know it you will start to sound like a really creative player instead of someone who just plays notes out of a tin.

Have a listen to the solo in Steel Dan’s “Reelin’ in the Years. Listen to how the notes dance up and down and in some respect start to live a life of their own with regards to the backing of the song.

Have a go at trying a variety of soloapproaches on one of your favourite guitars. Once you get happy with the notes you produce why not try some of your favourite effects, this will open up new sonic possibilities.
Hope to catch again soon.
Eddie