Tempo relative/absolute

Could a viable BPM option be absolute and relative like user and offset are with velocity? A BPM could be set to be +beats or -beats of the previously set BPM rather than an absolute value. Note value of BPM would be contingent upon whatever was previously set or if not set it would be the default at the beginning of the score. I don't know if this is a decent idea or not: it basically stems from the lack of rit. and accel. implementation. If this option were available, one could set or copy and paste a +/- 5BPM every beat easily to give a flow to the music. Just thinking. I wonder if this would even be usable once an acceleration/retardation system is in working order.