It seems to me that clean 3.5 and 2.8F's have even appreciated in value over the last few years, while a lot of other MF gear hasn't.
Rolleis are certainly great, but if you're happy with your current TLR why not broaden your knowledge with a different format or style of shooting (e.g. Large format or an MF rangefinder or slr). Shooting different systems has always been very rewarding to me (albeit frustrating while you're on the learning curve). Best of luck either way.