I think they need to do something new with the existing post-TNG/DS9/VOY warp speeds. Warp 9 is fast. Warp 9.5 is bloody fast. Warp 9.9 is hellafast. But now we're in Warp 9.975 and such territory. And recently in Trek literature we're into quantum slipstream drive. All of the Warp 9-plus numbers are getting blurry. I think even the slowest Trek ship can go, like, Warp 6. Time to redo the scale again?
I concur with this. I think a linear scale as well, based on some metric of velocity, whether it be light years per hour, parsecs per hour, or whatever -- regardless of the underlying technology involved.
So whether a starship is using warp, ultrawarp, transwarp, slipstream, or whatever, the Captain just decides how fast he wants to go and says "Hyperspeed Factor 14" or whatever, and that's like 140 light years per hour.