There are loads of ways to solve the power source issue, but before that point, there need to be a bunch of deign considerations for how to route the electrical connections on the figure itself. That's the most tricky part, IMO because it can quickly take a base body from generic to custom, which isn't good for the bottom line.
You can run wiring/connections outside the body and under the clothing more easily for some figures and that would be a way around making custom considerations for the plastics, but it can't work for everything.
At the far end of the problem spectrum is a figure like HT's various Iron Man iterations. Some of those have many separate battery compartments even though the bodies and parts may only be for that one figure. That's a complicated one to solve because with lights on the extremities, you'd need multi-conductor couplings for all the joints if you want to be able to take it apart - at the very least wires would have to pass through the joints. Glad I don't collect Iron Man.