The main limitation of the trap bar might be your legs hitting the bar when doing single leg exercises or the bar running into things like the edge of the box. Emphasising single leg exercises is a way to lift less weight, strengthen stabilizers and get more symmetrical recruitment overall.
A low-tech/cost way to make the box step-up more challenging and perhaps more specific to inclined terrain is to add a band; around your waist to an anchor behind you (see attached pic.) This demands more posterior recruitment throughout the ROM. It can be used for lunges (forward, back and side), split squat jumps, etc. Experiment with it but using a band should give more challenge.
Hope this helps.