Stick with 6x6 or try 645?

[lxdude]

I suggest sticking with your 124G for the time being. You love it. It gives you the WLF you want.
Buy, use and process lots of film with the money you would spend on a Rollei or 645. Wring out of the 124G everything you can. Watch yourself produce better and better images. After a while you will know for yourself what your next step should be. It might be the Rollei. It might be a Mamiya TLR with interchangeable lenses. It might be a 6x6 SLR like a Hasselblad 500 series or Bronica SQ series.
As you like a WLF, I don't think a 645 is the answer for you, unless you just shoot horizontals or use a tripod. You could use a 6x7 with WLF and crop to vertical, with a Mamiya RB/RZ 67, or Pentax 67 or Bronica GS-1, or with the Mamiya or Bronica you can use a 6x6 back. But it sounds to me like you should just stick with 6x6.

 

[MattKing]

Beware though, it's nearly impossible to find an affordable quality 6x7 projector. I shoot a lot less chrome now because of that.

That is the main reason why I have a couple of 6x4.5 backs for my RB67.

 

[fmajor]

I agree with the others. Since you're going to be shooting 6x4.5 and you're already shooting 6x6 you might as well stay with the 6x6 and crop a 4.5 out of it. Plus, you will develop a more intuitive "hand/eye" with YashicaMat - becoming a better photographer as a side benefit.