I think I'm going to try to get into duel this year. Always wanted to try it but just never did. I'm kind of tired of relying on teammates to actually play the game correctly. At least if I lose, I know that its on me and not because someone decided to troll or just doesn't know how to play.
I'm not a big fan of Duel simply due to how big a factor character selection plays in that mode is, would love a Duel mode were both players would be given the same randomly selected character and have mirror match duel.
Also with Season 4 being here across all platforms with a bunch of new/changed items I figured I'd share my custom builds in case anyone new wants to have look (and maybe encourage others to share theirs):
Hunters (Auto Attack and Ability Based): Not too much of a difference between the two you can also go with a mix of Transcendence start into Crit with Wind Demon/Death Bringer on characters like Cupid/Chiron. I'll also go for Fatalis in place of a Crit item on certain gods who can be less mobile (Ah Muzen Cab/Artemis).
Warriors (Auto Attack and Ability Based): Gladiator's Shield into Frostbound seems like a nice way to start auto attack Warriors to me right now as you get some nice cheap power/cooldown/health/defence as well as the ability to stick to a character. For Ability based characters I'm currently going more tanky and getting the cooldown as you generally want to get in, use all your abilities and try to get out.
Guardian (Ares, Damage Focused, Support Focused and ones with Heals): Ares now has quite a few more Aura items to build and I feel Sovereignty/Heartward Amulet are still essential, just a difference of when yo build them depending on the type of Guardian, I like to go into some defense/pen items early on Guardians that have good damage output.
Mages (Standard, Slow Required, Healing, Auto Attack and Tanky): Mages have a ton of options but feel like I've done a ok job of adding enough items from different trees so that I can access most of the stuff should the need arise... I still like the Dynasty Plate Helm for early damage/protection along with a stacking item (Book of Toth/Warlock Sash depending on if you want damage/health) on most standard mages. On Mages that work well with slows Gem of Isolation is a great item that provides a mix of health and damage, and similarly for mages with a heal Rod of Asclepius does a similar all in one job. The Emerald Ring tree is really good for auto attack based characters and Ethereal Staff/Spear of Desolation are now much more potent items overall and can fit into many late game situations.
Assassins are the class that I'm currently trying to wrap my mind around in terms of builds, as they can be quite diverse, you go from your more standard pen/protection characters (like Thor/Bastet/Susano) to Guys that can be built with power, attack speed, crit and so on...