I happen to come up with ideas like connecting tone cable to other 4 string instrument like cello.

in fact, I  just tried to do it using piezo pickup and even though I can not tune my sister's cello to play

ordinary guitar songs, I think this is possible if anyone make CDLC songs for the cello.  

 

if there anyone tried to do something like this, plz share your experience. 

I wonder that what kind of problem I will face, if i make one.

It should be able to play the session mode this way, atleast the producer said that beforehand.

The only thing I can think of, if you make a CDLC of another type of instrument (cello), RS is always going to take you into the tuning area for the first song you play and is going to want to tune it for either a 6 string guitar or 4 string bass. Without getting the tuning correct it won't allow you to continue. You might be able to tune with a bass first and then plug in a cello. I'm not sure if you could change the tuning in EoF enough to make RS think your playing a differently tuned bass.