Telluria can double as both healer and a tank, both offensively and deffensively. I honestly think this is an awesome feature and should be preserved in the balance. That means not touching the hot, minions, stats, class and mana speed. Her hot is good as it is, the minions will secure some indirect heal in case of a dispell and a slower mana speed would hurt the use in offensive setups. Her class and stats would maintain her as tanky as it is now.
That being said, we get to her offensive traits. As many pointed out, the way to go here is reducing the mana slow ailment and the initial damage. How much of a reduction in these effects is subject to discussion and a matter of crunching numbers.
That way, attackers facing Tellu would be encouraged to bring just a dispeller and not also a cleanser as it is right now. She would still be a strong tank (on par with Black Knight, imo) and still a decent healer for most occasions.