It’s the holy grail I think. All the groove boxes seem to have pluses and minuses.
I currently have a MPC live 2, and I love parts of it, but also share your view on some of its live improvisation limitations.
I previously had the Force. I think that is better for a lot of live stuff (assignable macros etc) BUT I felt the workflow was a little more cumbersome than on the MPC for building beats.
MC707 was good, as someone has mentioned. Though I wasn’t bowled over with the build quality and felt the workflow was a bit clunky. The new SH-4D could be worth looking into??
I think the elektron stuff is good at this, maybe syntakt though I’ve not tried it yet. Also if you’re willing to learn it I think the Octatrack is a beast for this…. I have been sorely tempted, but I like the breath the MPC gives - it has the pads, the inbuilt synths, it can sample easily, chop quickly, control midi etc.
Other option is a looper… RC505 mk 2 and a small modular system could go a long way live.
I guess the genre you are looking at is also important, whilst the MPC can do it all, other gear is perhaps more focussed/better for a specific style.