When people upgrade from Thiel

i suppose it depends greatly on what a thiel owner is seeking in his upgrade

if you like the sound and have a smaller model, then moving up the range may well make sense, room allowing

I had Thiel CS3.6's and then CS5i's and could never get a good soundstage and dynamics.

Currently running Klipsch Forte iv's and Zu Audio Soul Sixes, both of which sound more live and dynamic to me. 

It is nice to not have to deal with heavy, high current, and high power amps. Currently using a First Watt F7 or CJ LP66s, depending on my mood.

I own a pair of 2.4 and used them for about 10 years. They are in a second system now. I upgraded to Revel 228be after demoing a bunch of stuff priced from $10k-$30k. I use duel subs with all my speakers. 

Ultimately you want to pick a speaker that is going to deliver more of what you love about Thiel.  I would specifically want to know what your preferences are.  
 

That said, Kef, Focal, Marten, and Vivid are common upgrades with Vivid delivering one of the more pleasing sound profiles.  No secret I am a Vivid dealer and am a big advocate. I personally am a fan and it is one of a few brands I listen too at home.

That being said, the new Kef Metas are outstanding and one that you would be wise to look at.  Marten is also brilliant and one of the better sounding speakers I have heard.  I am not a Kef or Marten dealer.