Many things. The first to come to mind are:
Deliberately using a different playstyle more regularly. I like to play it up close with shotguns, pistols & melee and to rush spawns. I´m a bit too much set on regularly using only 2-3 starting points on each map. At the beginning of each wave, I look at where my teammates are and then rush towards the most likely spawn area, using mostly routes and access/RHA points that have worked well before. Given the limited number of maps, this sometimes makes things too repetitive.
Using more characters that I found boring to play and still make them work and fun to play. Something like a sniping Juggernaut sounds terrible to me, but I´m sure that it can be fun if played well.