So you've actually set yourself up well by starting with Kaeris - she is a 'do anything' sort of master. There are very few strat and scheme combinations where it will feel like you can't use her and have a shot at winning. That said, there's typically a better option for any given scheme pool in Arcanist. Think mostly about what how you want to play, and get the master that most suits that style. I love playing Colette crews, since they have a plethora of tactical positioning and sideways thinking. If you're more interested in getting into a toe-to-toe slugfest, then best bet is to go for Marcus. Want to try summoning? Ramos is the most efficient summoner in the game, bringing in up to 3 models in for one AP. Sandeep allows you to try a slightly more traditional summoner, with a decent list of models to summon. Rasputina is the blaster master, with ranged attacks and blast markers everywhere. Ironsides is the tank of the arcanist masters, and can be fun to pull off some crazy combo strings. Lastly, Mei Feng is the hit and run master, although most often her best move is to stand still and throw up a smoke screen.
On the surface, Mei Feng looks like a good choice, since she can also exploit burning, and she and Kaeris have a fondness for some of the same models (Rail Golem). Personally, if you are looking for cross-master usefulness of models, I think you're best to go with Ramos or Ironsides, who both beef up your ranks of M&SU options. But if you really want something different, go with either Marcus or Rasputina. Biggest drawback with Marcus is that soon you'll find youself picking up all the beasts from all the factions to try out with him.