Funny your example about the giant amoebas occasionally trying to ram you -- they already do that. However, it's not accompanied with a shift in speed, so it may be hard to tell.
Generally speaking having multiple boss behaviors is something we want to explore, and we already have the framework in place for it as a generalized concept, so it's not terribly hard in most cases as long as we have logic that fits within that framework (which includes attack types, move speed, damage dealt, changing forms if need be, gating by tier if need be, etc).