Diggle has been working with Oliver longer than anybody else, and the two had developed a bond that made them like brothers. However, that was broken last year when Oliver kidnapped Lyla and left baby Sara alone, with no sign of reconciliation in season 4 (yet). It is likely that the friendship will be restored this year, and the sooner Diggle can learn to trust Ollie again and reignite their bromance the better it'll be for everyone. Diggle also has his own mission separate from Team Arrow as well, aiming to get revenge on H.I.V.E. for the death of his brother. That's likely to lead him into conflict with Damien Darhk, and Digg may not come out of that on top. It'd be a shocking moment, and also an extremely poignant one, if he was to be killed just as his relationship with Oliver has been repaired and he's developed his own vigilante identity.