Maokai's ult shouldn't be centered on him

I mean, yeah. Sometimes you had the enemy walk out of your ult and move the fight. But that meant they spent a bit of time concentrating on repositioning their team instead of killing yours, and the CD wasn't very long - easy enough to wait out before re-engaging.

But as Mao, I usually want my ult covering other members of my team more than myself; I'm tanky enough that I can survive a lot and would love them to target me. Most of my team tends to need the survivabililty help more than me. And I'm often not right next to my team. Close enough that my ult can often reach both of us, but only with me on one edge and them on the opposite. It's not at all common that I'm standing outside of my ult, and I'm completely okay with that.

So now, instead of being able to protect my teammates and give the opponent reason to focus the tank (me(since I'm not in the ult)), I force my teammate to stand right next to me and get wombo combo'd.

(As a side note, you're also removing counterplay. Right now, if you can move the teamfight, you've got a 12-40 second window. This seems well in line with Riot's stated design goals.)