always wondered what it'd be like with kangaroos roaming the plains with lions.
Short. And messy ;-)

Actually it could be very interesting there for a while. A male red kangaroo is very big, very fast, and (when cornered) very dangerous. They have powerful legs with large claws. I'm from country Australia and I have friends whose hunting dogs have been torn apart by reds.
I'm sure the lions would eventually adapt to hunting such prey, but in the short term it may be the lions coming off second best. I guess they could start with wallabies and work their way up to reds...