My initial thoughts of mekko
First off, he's a great character design, and quite fun/funny. As for gameplay though, Lifeline is a brilliant and hilarious ability, love it. Surge is also a pretty damn good ability, giving him mobility no-one in the game has, not even Cass swoop can match it, but the cooldown on hit mod seems rather strong and you can have surge up 100% of the time if you are in a long fight, so probably needs toning down.
And then there's puddle...... While a zone control ability like Kulevs totem isn't a bad idea, puddle itself is too strong. 50% damage reduction is crazy high, coupled with the 20(?)% lifesteal it just gets absurd. The duration mod allows you to have the puddle down 100% of the time, the size mod allows you and your team to fight in a larger area, and the extra lifesteal just makes the sustain inside puddle even more bonkers. All of this coupled with the fact that puddle can only be removed from the map by killing Mekko, makes it a bit obnoxious in my opinion.
I would suggest either a hp system, like totem, so it can be broken by aoe or focus on it, or maybe have it disappear if Mekko leave's it, so people can work around and try to deny him of the area to get rid of it. I do like the puddle, it is an interesting mechanic, just feel it is a bit over tuned, maybe lower the damage reduction and lifesteal it inherantly provides, and give us some sort of counter play to it other than just "murder Mekko".
This based on a day of playing, and a night of watching Mekko be played on stream.
We greatly appreciate your feedback and suggestion. We'll make sure to keep track of this.