What kind of organization are you working in?
Why wouldn't IxDs move in the same trajectory as your other creative roles?
Shouldn't an IxD get the opportunity to direct design projects the same as an visual designer would be considered?
We are an internal team and we use the following, but I don't think it would work for you based on what you said above:
Jr. IxD, Staff (unstated, actually) IxD, Sr. IxD, Lead IxD, Team Lead IxD (playing w/ this), IxD Manager, Sr. Manager, etc.
Something I would make sure to do, is not create any differentiation between your visual design track and your IxD track. Any perception of difference is going to be twisted into a perception of devaluation among your team (that goes both ways, btw).