If I understand the runway geometry you describe, I think only a very brave person would not consider wake turbulence separation.
If the arriving aircraft is No 1 and lands - then departure goes, no WT sep required
If the arriving aircraft is No 1 goes round - departure stays on ground until WT sep exists
If departure is No 1 - then normal visual separation except that if the arrival goes round you better have an ace up the sleeve and issue instructions that does not compromise WT separation. Once you are dealing with two airborne aircraft a distance standard is likely to be easier to apply than a time standard.
Why dont you build an airport with runways that cross in about the middle so you could expect one or both aircraft to be on the ground through the intersection. Oh for life to be that simple.