It's time for the best time of LoR... the deck theory crafting.
I feel like we can make most decks that the expansion has to offer minus the Shurima all in Sun Disk deck and whatever deck the unknown champion fits in.
Here's what I have thus far:
Renekton + Darius Overwhelm
Nothing fancy here, just good ol' fashion smorcing. Utilizing Shaped Stone which is an incredibly powerful card for an aggressive deck and for that we are using some Landmarks. The Roiling Sands are pretty good for this deck because being able to grant crappy enemy followers Vulnerable is good for most of the deck especially for Renekton.
Renekton + Sejuani Overwhelm
Basically the same deck just with Noxus cards swapped out for Frejlord. This one has a huge hole in the 3 drop slot, there aren't any good 3 drops to run here and we are basically forced to use something like Mighty Poro here. Battlefury instead of Decisive Manuever for the open attack finisher. Quick Sands taken out for triple Troll Chants as that card is just too godlike.
Now this deck is spicy.
Watcher Retreat Return OTK
It's a Lissandra Watcher OTK deck on turn 8. Basically the idea is to use Retreat + Return on Ice Pillar to burst out a Watcher and end the game on the spot at turn 8. The rest of the deck is similar to the Feel the Mina deck that was used in the previous seasonal tournament. Deck also features some good targets for Retreat Return like Shadow Assassin, Concussive Palm and Tavern Keeper in case you don't have the combo.
Traditional Freljord + SI Lissandra OTK Control
This one is much more traditional deck featuring Lissandra. Pure Control game plan, win with Ledros Atrocity combo or Lissandra OTK. The OTK here involves Fading Memories into Ice Pillar for Watcher and can also be used on Ledros.
Undying Kindred
The best competitive deck to play Kindred is going to be Go Hard with Kindred (hence why there is only 1 . However I think there is a lot of synergy between Kindred and Undying decks. Most of the kill your own unit stuff procs Kindred's marks. Going Shurima splash for the Vulnerable effects and Spirit Fire being basically a better Withering Wail in most cases.
Taliyah + Lissandra Landmark
This is the best I could come up with for a Taliyah deck. It has the alternate win condition of Frozen Thrall into Draklorn Inquisitor. Generally just stall the board early, try to flip Taliyah then go to town on them. Keep units alive with Ancient Hour Glass and Troll Chant. Use Roilling Sands to challenge enemies with Taliyah. If game goes late then your Thralls flip and Lissandra too. No idea how good this type of deck is going to be. 3 Desert Naturalist is going to carry this deck with the natural combo of Lissandra into Desert Naturalist into Taliyah on some landmark.
Riven + Sivir Allegiance
I think best Sivir deck is just going to be a standard Noxus overwhelm list where you are just swapping Champions around. This one is a bit more interesting in my eyes as it takes more advantage of the level up condition of Sivir granting keywords and Riven's inherent value of adding keywords into the mix. You have more spread out threats with Ruin Runner, Sivir and Riven being good buff targets for Blade of Exile. Went Allegiance route since I needed another juicy 4 drop into the mix. Whirling Deaths are in there in order to try to level up Sivir quicker if possible.
Jarvan Elite
The quintessential midrange deck featuring Elites. Plaza is pretty essential for the deck as are the 3 Penitent Squire to get pick offs and enable Jarvan. I am not running the new minions because they are too conditional with out that much pay off. Cithria the Bold and King Jarvan 3 as finishers. Golden Aegis as a better Rally for this deck. Bannerman because might as well. Brightsteel and Grizzle Ranger are too strong cards not to include even in an Elite deck. For 2nd color splash, Shurima is pretty cool with Renekton and the Vulnerable spells however then the build goes in a different direction.
Greenfang Warden looks like an insane pick here but it actually makes a lot of sense in this deck.
*Elite tag so there are synergies in play. Curving from Battlesmith makes this a formidable 3/3 with Divine Shield and Scout.
*Strong with Grand Plaza, can pick off two targets.
*Jarvan can use his Cataclysm skill on Greefang to get a Rally trigger going since Greenfang is a Scout.