My views on general Fade-in effects:
We have two possible sources of music:
- professional mixed down
- self recorded/created music
professionally mastered music has its own Fade-ins already applied. In general, those ar very short, just long enough to avoid an introductory click.
Why is that so? It's simple, because a musical piece wants to hook on a signature lick from the very beginning. You can't fade in Beethoven's "tatata-tah" neither you can do it with with "Can't get no satisfaction".
It is the saying that your solo is only good when you hit the right last note, no matter how Van Halen-like you played before. This applies also to the beginning of a song. It is important to establish the groove as soon as possible.
Therefore, if you want to kind of fade in music, the rhythm section (especially the bass) should be left out. It's far more musical to introduce more melodic and harmonic content over time than just fade up the whole song.
Of course, the opposite can also serve as an interesting effect. You introduce the song with some pads that play the melody for a few measures (or just some Sfx like wind, helicopter birds etc.) and then the full rhythm section joins in. In this case, your possibilities to apply a fade-in on commercial made music are very limited, either you leve the whole thing as it is or you make a very short fade-in towards a significant section of the song.
For your own music, there's a lot more freedom. It is in fact the creative usage of the fade-in that makes the difference in the end.
It is no surprise that the music of the beatles late sixties albums had had such a huge influence on the whole pop music. They played a lot with their tapes and introduced for example reversed sounds.
If you start a song with an reversed cymbal sound, you'll have an creative fade-in effect. Although it is somewhat overdone today, particulary in synthisised music.
What can we learn so far?
- Long fade-ins are almost exclusevely used on pad sounds and sfx, where you can apply the same types as with the fade-outs. However, the attacks shouldn't be too slow (no linear fade-in in general).
- Rhythmic music tends to have a very short Fade-in (linearly, over some ms).
- Songs go towards a climax by adding more melodic content over time.
- Fade-ins work rather with the harmonic content than with the volume knob (like opening a wah-wah for example)
- Strong bas or snare sounds shouldn't be affected by the fade-in.
- The fade-in should respect the rhythmic structure of the song. It's better to fade-in over a copy of the first measure(s) than to attenuate the introducing riff, if you don't want to end up with a rushing intro (11 and a half bar blues...)
- Special effects are widely used for Fade-ins: reversed sounds, filter sweeps (see the second part in Madonnas American Pie)
- Often, the different instruments are faded in differently. The rhythm section starts full and the melody section gets in gradually (or reversely).
- Fade ins can also be applied to the stereo image: from left/right, from far away or narrow towards wide.
- There doesn't exist such a thing like the "general" fade-in effect, not even for songs within the same genre or style (except the very short linear one).
A one-click Fade effect has to be much more complex (or simpler...) than its fade-out counter part. the simpler possibility is that the effect always applies a short fade-in, no matter how long the selection is, but this should be expressed by the effect's name.
The more complex possibility could be that the effect analyses the selection it is applied to. Selections under about 0.1 s are linear. On longer selections, it depends how diverse the source material is. Pad and string sounds (i.e. legato music), where the rms (or peak) curve is smooth, can be faded up gradually. Rhythmic intros, of which the rms curve varies a lot, should fade up much faster.
I am afraid that even this long reply won't help you much on your search but it hopefully gave you some new points of view.
I hate this idea icon, anyway.... 
