There were rumours that a 68050 part was designed but then scrapped. I don't think they are true. Motorola seemed to use the even tens increments for major changes to the 680x0 series and the odd tens increments for smaller changes.
Anyway, don't forget the 68070 (although that's by Phillips and definately not an upgrade to the 060).
There's also the CPU32 line which are interesting in their own way, and of course the coldfire series.
From a programming perspective, I like the 68020 as a starting point. The scaled index addressing mode (aX,dY.w|l*N) is very useful although on the 68020 you are just as well to calculate it yourself unless you are short of registers.
Other stuff added to the 68020, such as bitfield operations are also interesting but not a great deal of use due to the number of cycles they take.
