Yeah, all doombots are regular champs with modified abilities. Also I think better bot AI, not sure if regular bots are that good with Zhonya.
Say Annie (FUCK HER FACE WITH A HAMMER) for example, she has a passive which gives any nearby tower a sunfire aura that damages the enemy team (and by any tower I mean any tower, you cannot towerhug with her nearby). Her skills also have AoE and Tibbers is now fuckhueg with more HP and his every attack causing knockback.
Lux for example fires a bloom of ult lasers like Shadowgandor said, but that's not all, her snare (just like Morganas) splits into like a star of five or six snares whenever it hits anything. Morgana additionally has a snare circle around every pool she casts.
Malz has a ton more voidlings now and his W summons another five or six pools around the inital one.
Fiddle is also a fucking bastard. He has clones which ult randomly out of bushes ANYWHERE ON THE FUCKING MAP!! Sure they die easily, but holy fuck you can never be safe in the jungle, never.
Karthus also has his passive randomly activate if he takes damage, that, combined with Zhonya makes him stupidly hard to kill without dying yourself.
Veig randomly rains a bunch of dark matter around himself.
Cho's ult now goes up to 8 stacks (was it six before?) and with each stack the AoE on his abilities grows (as in, after several stacks rupture fires off atleast three zones in a line).
Wukong summons copies with most of his abilities now, and they all knock up when he ults.
Ziggs is super fun too, his abilities now splinter all over the place with fragments homing in on nearby champs. Zyra too, she randomly summons extra plants whenever she casts anything.
Also fun is Galio and his random taunt zones, which means that you have to watch out all the time, because they appear anywhere in his general area, sure they can be avoided, but if it's crowded it gets pretty damn hard to do so. Also his other abilities now have more shots. Tornado thingy summons three parallel ones, so he covers the whole damn lane with that, and his concussive blast fires off several projectiles in all directions.
And all of this is just the first difficulty (there are three), I'm not sure if higher ones get more broken shit or what but I didn't dare try level two.
Also also, friendly tip, get good pusher AP carries or anti-AP carries since the bots are usually atleast three AP carries and two tanks.