The next step would be a 4 channel mini helicopter. Still I will focus on models for indoor play and use. Keeping in mind that the requirements have developed since 3 channel coaxial helicopters it is difficult to find adequate and challenging products. Still valid are the general hints concerning the LiPo accumulators. They will increase the lifetime of all models.
There are several options for 4 channel helicopters. The first and most easy option to build is an avatar style heli with additional motors and blades on each side.
More advanced models use a swashplate to control forward and backward flight. Like the following models:
Simulus GH-424 2.4 GHz
click to see more pictures and reviews: Simulus GH-424 2.4 GHz