Coming from a yashica, here are some good options for you:
1. A Rolleiflex with a planar lens would be a nice straightforward upgrade and would not cost you too much. Many people rave about the quality of the Rolleis.
2. you said you wanted more lens options. As mentioned before a mamiya TLR would fit the bill. the C330 has all the bells and whistles but i found it was excessive. I prefered the C33 with auto shutter cocking but otherwise simple design (and can be had for a song on ebay!). However both of these are heavy and clunky compared to the yashica. The C220 is much lighter that either but you have to cock the shutter separately from the film advance. There are alot of lenses out there and they vary widely in date of manufacture, amount of use, and thus quality i suppose. i havent heard too many stellar reviews of the optics either. But im not a regular user of them. I have a c33 i dust off every once in a while.
3. i wont go into the various MF SLR's, but suffice it to say they are all bigger, heavier, and very expensive. The mamiya is the biggest (ITS FRIGGING HUGE!), but also probably the cheapest for a system. The mamiya is reliable, straightforward, and good quality though. Again, people dont fall in love with the lenses except maybee the latest KL's. The 'Blads are beautiful, compact, have wonderful optics, but are the most expensive of the bunch.
4. Dont count out the graflexes either. they have many lenses available or even adaptable. The rangefinder can be adjusted for any one lens (takes a couple hours but its dead-on accurate) The common lens on them is a 101mm raptar or ektar which is ok, some have an 80mm f2.8 Xenotar which is a fabulous lens (i have one).
5. Another option is the rangefinder cameras including mamiya press, mamiya 7/6. graflex Xl, koni omega, and Fuji. These all have good quality lenses and may have a good option for you in one. I havent owned any so i will not comment.