Hmm. If that's the case, I'd probably push for a different direction than higher-damage mage. If the team is running in and dying, having another squishy mage running in behind them won't do a ton. Maybe with Brand, Annie, or Lux you'd be able to pick off a kill or two, but realistically it won't matter much if the rest of your team is already dead. The survivors on the other team will likely still take you down after you throw out your combo.
I feel like something that's able to disrupt the other team in one way or another might be a better option. Janna is great at disengage and peel, though your team might still not follow and actually disengage after a good disengage. Nami has her ult that can make for a solid interruption of the other team, and her bubble is good for peel (just choose targets wisely). Then there's the tankier route still available. Braum can jump in and block spells and slow lots of things. Leona has a lot of easy CC available to interrupt things. Ali can dive in and knock things around.
I also like the Sion pick as a tanky disrupter, too. Especially if you've already got some practice with his kit in general. Don't expect to get away with quite the same level of survivability you might be used to out of top lane with him, but you can still jump in and cause chaos.
The nice thing is that these skills will still translate for the most part as you are playing with better/smarter people. You basically would switch from being reactive with all your spells, to being proactive with your engage. Generally, you've got the best shot at winning a fight if your team engages it (well).