Formal:
* Shortly
* Presently
* Imminently
* Forthwith
* In due course
* In the near future
Informal:
* Quickly
* Shortly
* Right away
* Any minute
* Before long
* In a jiffy
More specific:
* For a specific time: "Tomorrow", "Next week", "Later today"
* For a general sense of immediacy: "Immediately", "Instantly", "Directly"
The best synonym will depend on the specific situation and the tone you want to convey.