All of the technique you can learn, all of the scales you can memorise, are like acquiring a large vocabulary of words and phrases. None of it is of any use unless you actually have something to say.

What sticks with people is the melody of a song. Along with a strong lyric, the melody is a very big part of what a song is saying to the audience. Borrow some advice from public speaking:

"First you gotta tell 'em what you're gonna tell 'em."
"Then you gotta tell 'em."
"Then you gotta tell 'em what you told 'em."

Your solo can fill any one of those functions, but it must do it clearly, to be effective. You can present some variations of the melody, or provide a bridge between two sections which feature the melody.

Another concept that gets forgotten far too often, is the fact that if people can't hum a solo, they probably aren't going to want to... which means they will have forgotten it before the rest of the song is even over!

So don't work too hard at it. Just feel the song, try to express what it means to you, as clearly as possible. A solo should be more than just a few bars set aside for you to show how much technique you have learned. It should be an integral part of the song, a place for you to enhance the song. The music is what counts. If you make it sound great, you will sound great!