How can one change the wingtip geometry to enhance the wingtip vortices?
You are not interested in wingtip vortices. For now you are barking up the wrong tree. For that tug idea to work, you need to focus on the trailing vortex which is a result of the whole wing, not the wingtip.
How to maximize trailing vortex intensity? Fly slow, use a small aspect ratio and create lots of lift. This means a heavy airplane with a high span loading, flying close to stall. Niceties like the spanwise lift distribution are much less important; all low aspect ratio wings will show an elliptical or nearly elliptical lift distribution anyways.
Next, position the following airplane right. Between the two vortex cores of the leading airplane you will find severe downwash. What goes up must go down. This is not only true for aircraft, but also for the air surrounding them. So any part of the following airplane needs to stay outside of the immediate wake. In order to maximize the recovery of the energy wasted in the leading airplane you need two following airplanes, one to the left and one to the right, and ideally both need to be specific to their side since both will fly in an asymmetric flow field. If you don't account for that by optimising wing twist and weight distribution, your rudder and ailerons on the following airplanes will do that, albeit less efficiently, just in order to stay inside the upwash part of the wake.
To answer what you asked: For maximum wingtip vortex intensity you need a steep gradient of lift at the tip. This means a spanwise lift distribution which peaks near the tip. Of course this is what a good airplane design will avoid at all cost, because it not only makes it less efficient, it also results in dangerous stall behavior.