Unless they do some bigger changes, they will be pruning every expansion to fit new abilities in.

A few things they could do to fix this in the longer run,

-More traits, they started to do this with SB, but hopefully they do this more moving forward.
-More pruning. Some abilities serve little purpose but still warrant a spot on the bar. Some examples are Heavy Thrust on DRG (you only hit it every 30 second to keep up a buff) or even more so their combo set-up WS (Impulse Drive, True Thrust, Vorpal Thrust) whose sole purpose is to be used so you can use better WS for higher damage (Full Thrust) or debuffs (Disembowel, Chaos Thrust). Though pruning them would mean more of a removal of the combo system which I doubt they will do, so alternatively for these abilities they could gie them the PVP treatment.
-Do what they did for PVP. This is where a big change would occur. For many abilities/spells you only use them in combo or with certain procs. So they spend a good chunk of time cluttering up the hot bars. RDM I think is one job that would highly benefit from this, as most of their abilities could be condensed down to 4 buttons (Jolt/impact, black mana spells, white mana spells, melee combo). The usual argument against this is "But hitting 1-1-1 is boring/too easy", though I'd counter argue that 1-2-3 isn't much different, and all that added bloat just adds a false sense of complexity. In the case of RDM they could condense them down to those 4 buttons for their main rotation, which that opens up 7 new slots for new abilities that can give more depth to the job without having to remove/rework everything they already have.

Of these I think going the PVP route for buttons is one of the better things they can do. They can condense redundant skills into a single button without removing them from the game or doing big reworks to how the class plays, and open up more slots that can add to the job as it already is without all the time consuming reworks it would take to remove/rework most of a jobs kit.