Some thoughts on characters I have played. You can theorycraft with many champs and they seem pretty broken in one aspect, but end up falling way short in another.
Garbage Tier
Singed
Yes, I know that permanent R mode sounds cool and all. But his primary source of damage, poison, does not increase at all in this game mode. But the biggest offender is that Singed can't outrun anyone anymore. A champ with any dash is now speedy gonzales (an overrated trait in URF I believe, but it makes Singed shit in comparison).
Low Tier
Lux
Here again the shining light is the ultimate, a now beautiful 10 seconds. Unfortunately, Lux otherwise gains very little from this mode. Lux is a calm, tactical character. She just can't function under this level of crazy, and forget using her passive outside of a duel.
Mid Tier
Leona
This was the first character I played, so I couldn't know going in what her shortcoming would be. Every character has 25% tenacity, so not only is she not a perfect lockdown, she can't even combo her R with Q well anymore, especially when you consider all the people who are going to be blinking out of every lock. Stil, she has perma-W and offers about as much to a team fight as always, it's just not a tank friendly meta.
High Tier
Hecarim
I hear some people would say this character would be higher, and I think he's really damn good but there's surprisingly more broken characters. Not much needs to be said about Hecacopter, he is a rare good MS character in that he can Perma-E and it stacks. Any character that already had a low cooldown ability is bound to be good, and Hec does not disappoint.
Graves
I feel like Graves is everything that people think Ezreal is right now. Sure his dash isn't as good, but his Q is much more suited to this kind of game mode and his R is pretty sweet too. Between a 3 second dash, a redonc poke, and all the buffs that ranged carries got, Graves is very much a character that win with any amount of early kills.
LeBlanc
Like I said, she is overrated. I might be overrating her myself by putting her in High tier. There are just many things that make her awkward. The most of which being how the cooldown on your W will get lower than its own subcast. LeBlanc can solo very well early, and if you get fed your team has a strong chance of winning. But I feel there are better champs who can win for their team even without lots of early kills.
Sion
As usual, people are going to come to Sion for one thing and it's going to backfire. That would be his shield. Even though the cooldown is 2 seconds, you still have to wait all 4 seconds to blow it up. That feels like an eternity in this mode. So why the high tier? Simply because of his Q. Not only is it one of the premier lockdown tools of this mode, it also hits like a ton bricks. And even though you can't rely on your shield exploding, you can put it up almost immediately after an opponent breaks it. Even his E and R have late game significance in a mode with a ton of backdooring, not to mention that his R is up permanently.
God Tier
Sona
Allow me to play you the song of my people... QWQEWQEQWEQWEQWEQWEWQEQWEQWEQWEWQEWQEWQEQEWQEQWEQWEQWEQWEWEQWEQWEWQEQWEQWEWQEWEQWEWQEWEQWEQWEQWEWEWQEWEWQEWQEWQEQWEQWEQWEQWEQWEQWEQWEQWEQWEQWEWEWQEWQEWEQEWQE
RIP in peace
Soraka
Just... tear to my eyes beautiful. As a longtime (and sometime offensive) Soraka player, going 30/5 was just glorious. Her Q is already way stronger than Hecarims, but when you add in constant heals and the ability to neuter other purported "god tier" characters (here's looking at you LeBlanc, Urgot), Soraka may be the best this game mode has to offer.