He was reportedly a successful, if ruthless, military commander in campaigns against the Scots (including the crushing victory at Halidon Hill). So in that sense he could assist his brother in the HYW as a loyal lieutenant (he didn't show any inclination towards revolts/plotting OTL, and the siblings difficult upbringing may have created some solidarity between them).
I'm not sure how Edward worked things OTL, but John could maybe stay behind to govern England in his brother's absence (did he appoint regents when on campaign?). Given his early experience in the area John could also be told to handle the Scots.
The interesting question is who would he marry? English or internationally?
Are there any English heiresses Edward can snap up for his brother? Or any French/continental brides useful for support in the HYW?