sustained damage and high mobility are not always easy to find, as the hyper mobile champs tend to be assassins with burst damage. However, you could go Aurelion Sol (sustained damage, stun/knockback, passive speed increases), Kayle (high sustained damage especially with Guinsoo's, Hurricane and Nashor's, has a slow and a invulnerability ult for utility,only downfall is her only real mobility is a speed boost off of her heal I believe), Ekkos still not a bad choice as even if his abilities are on cooldown, his passive can grant some sustained damage or finish off kills. Lulu and Oriana would make good choices as well. They are utility mages who's strengths are in the AP scalings on their utility. Lulu has two different shields, an AA enhancer for your ADC, a slow, an AOE knockup and a polymorph. Not to mention that because of Lulu's passive, she can deal impressive sustained damage from Pix. Oriana, has an AA enhancer of her own AA while helps with some sustained damage in lane. She has a slow, a shield/speed boost a stun/displacement ult. Her sustained damage is little worse than Lulu but Oriana has a little higher burst and, if you go for a good amount of CDR, Ori can roll through her spell com oc fairly quickly to end up with a fairly high DPS Mark anyway. I think Lulu brings more utility to the party while Ori brings more kill pressure. If you want to solo carry a game, Oriana would the better of the two. If you wanna enable your team to carry, Lulu is the better option.