The other answers are great. I just wanted to give a list of what came to my mind immediately, if I were to hear a friend or brother say it:
(In parentheses are how they "feel" to me if I were to hear them)
- Ya casí llego (I'm almost there - feeling of your imminent arrival)
- Ya voy (I'm already on my way - feeling of you in motion in that direction)
- Ya vengo (I'm coming - feeling of you coming this way)
- Ya voy en camino (I'm already on the way there - feeling of you being in the process of traveling, more so than "ya voy")
Adding "ya" adds a sense of being already in progress, quick, very soon, or imminent.
These are subtle differences, but by comparing them you can get a better sense for how to communicate what you want in the way you want it to be heard. Be aware that for something like this, there are going to be some regional differences, as a few have mentioned.
(Spanish: Mexican, Guadalajara area. See profile for language background.)