TEMPO Adjustment Tool (not the tempo view)

Frequently while recording I encounter the problem of playing to the metronome when feeling requires otherwise but you still need to fit in the bars structure.
Tempo view is a very obsolete and awkward tool and good luck to everyone who would try to use it to adjust tempo in order to fit within bars and maintain fluent changes in tempo.
I imagine a tool which would provide a graphical interface showing bars with the prerecorded sound's waveform in the background.
The lengths of the bars (as well as their quarters if required) would be freely adjustable according to the visible waveform just by holding and dragging and based on those lengths changes in tempo would be calculated. important thing is that the changes need to be fluent and employ something like Bézier curves or some other clever algorithm.
Because I am a musician this is a musician's not mathematician's approach to the subject.
I hope this is clear enough what I tried to propose here.