I’ve considered Rider, and I like it quite a bit if we can make it happen on turn three. The problem is that we only have six lands that tap for white mana, and eight that tap for blue. We play a lot of swamps and forests, too. Seeing as we have until turn four/five to hit Chromanticore mana we’ll be able to cast it on time a much higher percentage of the time. It’s also worth noting that the current maindeck is quite strong against Lightning Strike and Bile Blight, while adding Mantis Riders would make us weaker to these spells.