So I accidentally started collecting TT.....

[Zggy91]

So I was going over my model collection and realized that I have as many TT masters as any of my other factions, but none of the smaller boxes. I have Misaki, Lynch, and am getting Mei. The question is how would I expand in a way that is complementary to what I have. Yasonori seems like a like a solid choice for any of them, but what about outside that? 

[tomjoad]

Just buy 10 Thunders Brothers. You'll be fine.

[Jinn]

You're going to want at least the TT upgrades from the first upgrade deck because that has Recalled Training, Misdirection, and a few other goodies.

The Shadow Emissary is the best Emissary, so that is a good purchase eventually.

Terracotta Warriors are one of the big advantages that the Ten Thunders have over other factions, so get them also.

[H4ml3t]

In addition to the above I see Low River Monks and Sun Quiang getting a lot of play with many TT Masters.

[Kogan Style]

Emissary first then depending on your position on Proxies - either buy the Shen Long box set (netting a very good TT Master in the process) or Pick up a proxy for Sensei Yu, who will really help all those Masters you mention above.

Mei is a tricky one to cater for as the models she can access cannot be used by anyone else. Mech. Porkchop is very good for her, other models include the Metal Gamin, Willie, and Sparks but I have found you'll rarely include these guys in a TT crew.

Yas is difficult to work with as it costs so much that you really have to consider how you build your crew to maximise its potential. 

Burt Jebsen makes it into a lot of my crews with Lynch and Misaki. He is another damage dealer that can also run schemes very well. Push and Fast (TTs big bonus) really speeds him up, giving you 4AP to work with most turns. He has no particular synergy with either Master (other than providing another threat for the enemy to deal with in melee which can take the heat off Lynch and provide some wonky interactions with Misdirection on Misaki) 

For Minions - TT Brother, Low River Monks are the usual inclusions, followed by the Illuminated (mainly with Lynch) and Katanaka Snipers when more damage pieces are required.

 

[OneLittleThunder]

You've gotten a lot of good suggestions here. 10 Thunders Brothers, Terracotta Warrior (you'll probably only need one, so find someone to split a box with if possible), the Shadow Emissary, Yasunori, Low River Monks, and Katanaka Snipers are all good investments. You'll also get good work out of some models you may already have - Illuminated, Graves, Tannen, and Depleted for Lynch, Woukou Raiders (and maybe a Jorogumo) for Misaki, Kang and the Rail Workers for Mei. That covers the majority of the models I find myself using on a regular basis.