We could fall back into the same mold of Temur Delver, Miracles, Shardless Sultai, and Esper Deathblade, but what's the fun in that? There are several options available to us, including Jace, Vryn's Prodigy. Aside from the Karakas issue, Jace is definitely good enough for Legacy. In order to utilize it, you want some proactive spells, and Thoughtseize is possibly the best among them.
Thopter Foundry is yet another exotic option that hasn't quite hit the mainstream. Chris Andersen almost put it on map on the Open Series, but fell a little short. Other than Abrupt Decay, there aren't many good ways to deal with it in maindecks. Pyroblast shows up here and there, but with Dig Through Time, I imagine that practice will wane slightly. If so, Thopter Foundry looks pretty good in the format.
In order to buy some time, this deck uses the Counterbalance/Sensei's Divining Top combo. It doesn't go super deep on stuff like Terminus, but extra Sensei's Divining Tops are nice to sacrifice to Thopter Foundry since you can tap it to draw a card and sacrifice it in response. Liliana of the Veil also plays quite nicely with the Top.
Overall, I like this option a lot. If you wanted to, you could even sideboard some Stoneforge Mystics and hope to get the people that side out all their removal against you, although that might not work when you're giving them a reasonable target for removal in Game 1 with Jace. Still, Stoneforge Mystic can fetch part of your combo, so it's worth considering.