So far the simplest assist I can think of would be a front wheel hub motor and a switch that turns on a constant amount of torque. The simplest: sure. However

So far the simplest assist I can think of would be a front wheel hub motor and a switch that turns on a constant amount of torque. The simplest: sure. However if you're used to cycling chances are you're not going to like that front wheel hub all too much. It feels weird overall (might depend on geomerty though) and has the tendency to slip on upwards sloped unless you do an effort of getting weight over the front. For my personal taste, hub in the backwheel feels the most natural.