It's pretty hard to control a lane with Xerath unless you really get fed in your lane, so your best bet is to level arcanopulse and fire it down lines of minions, especially when your lane opponent is there, and last hit where possible. If you try and duel lane opponents at full health such as Ahri and Annie you will probably lose, so play passively and farm until the teamfights.
I've not personally played Malzahar, however 2 of his moves are slightly delayed AoEs, one doing a silence and damage and the other doing damage over time. So you can harass and farm quite easily and your passive voidling minion will just pester your opponent when you lay malefic visions on them.
Ryze has incredible burst damage and his passive makes it easy to get insane amounts of ability power with items like Archangels Staff and Rod of Ages. Definately worth a go for 450 ip.
Karthus is one of those who is extremely dangerous even if he loses his lane, mostly due to his ultimate. His ultimate hits a set amount of damage to everyone on the map after a 3 second channel and when you're fed this can easily take half health off of most squishies.
Cho'Gath is brilliant, because of his ultimate. If you get a kill with his ultimate, he grows bigger and gains more health. Apart from looking awesome, it means that you can build him as a pure mage and he'll still be pretty tanky if you can hit 6 feast stacks. Definately worth a try as he is only 1350 ip. The trick here is to use his combo, use rupture to knock them up and feral scream to silence them while they're up in the air.
