My suggestions for a fourth keystone in the other trees
Conquerer has landed and has been a huge success for a ton of bruiser champions that have been lacking a damage option

We're still missing a fourth keystone in the other trees though, and hopefully we'll see some soon. These are just ideas I'd like to see.
Domination - Lambs to the Slaughter
Being unseen for at least 1 second causes you to accumulate adaptive damage and penetration over 5 seconds. This bonus builds up to a maximum of 15/25-40/60 AD/AP + 20 lethality/5-15 magic penetration. Once you're seen the bonus stats will decay rapidly over 2,5 seconds. Scoring a takedown before the buff runs out will reset the the buff to its maximum value.
Making good use of ward control and invisibility is the key to maximizing this keystone. It's good on champions like
that can successfully build up the effects, and then stay in the fight to maintain said effect, without being locked behind a time wall like Conqueror.
Sorcery - Lock n' Load
At all times your abilities with more than 700 range is increased by 10%
Successfully scoring a takedown within 3 seconds of landing an ability beyond 700 range causes your next 3 long range abilities to deal an additional 15% adaptive damage
Keystone meant for artillery champions, it's not as effective at poking as Arcane Comet, but if you are good at staying safe at range you can gain a lot more damage on your long range abilities. Keep in mind you MUST be beyond the range requirement to trigger the bonus effect.
This keystone is excellent on champions like
that want to do long range poking but can't reliably use Arcane Comet.
Resolve - Fury of a Demigod
Enhance your boots with Fury of a Demigod, causing your boots to become an activateable. While under this effect, your next hard CC (except displacements) effect is reduced by 30%, but becomes uncleanseable. Landing a second hard CC during this time will increase its effect by 30%. Only you can trigger this effect. Cooldown: 150 sec - 90 sec
This is exceptionally good on champions with lots of crowd control
and opens up a choice. You can either decide to use your strongest CC ability and make it uncleanseable, to hopefully get somebody before it runs out, or use the short window of opportunity to use a less effective CC ability to increase the power of your stronger CC.