Your main job is to deal with commons - clearing out large open areas (no horde) or holding a chokepoint while your teammates deal with mutations (During a horde).
I don't recommend Down in Front for melee builds - it's usually a people problem if you (melee) are dealing FF or if they are dealing FF to you.
In the first case, people should NOT be standing near a melee user - most people should be hugging walls if you're properly positioning, especially ranged teammates who don't wanna get hit by commons from all sides. Only Melee should be out in the open but rarely - if there's no hordes; if there's a horde they should be frontlining a choke point and it's easy to avoid FF if you position well
And if you do take FF as a melee, it's generally okay due to face your fears and vanguard generating so much temp hp. For solo queue it's understandable, but I really don't think it's worth the slot.
Sunder adds the biggest damage increase along with increasing teammate damage against mutations. That being said, the ONLY mutations you should be fighting or tallboy variants - Stinger variants can easily be dealt with by using ranged weapons or hopefully teammates are good at killing them quickly. Reeker variants have heavy drawbacks for trying to melee kill them and I highly recommend against it. Tallboy Variants can be taken care of with a Fireaxe or Baseball bat but generally you're the last person who should be dealing with them.
Meth head + Holly = never run out of stamina; add brazen later on to help with attack speed and stamina
Face your fears + Vanguard + Battlelust gives insane healing/sustain;
Numb (15%) + Motorcycle Helmet (+15%) + Spikey bits (+10%) + Holly (+10%) gives insane (50%) damage reduction allowing you to tank acid/volatile/fire zombies and bombsquad gives protection against exploders
Run like hell + Mad Dash combo lets you sprint jump away from hocker spit and hocker/reeker family if they get too close and gives a surprisingly large sprint speed/distance
Highly recommend running with tac 14 as it lets you stumble reekers and 2-3 shot stinger family with weakpoint shots (if you have to, preferably leave them to teammates - same with tallboy family)