Reducing ability bloat is easy and it's been done in other MMOs.

The simple method is this.
Example: Currently we live in a system where if you have 6 attack abilities they work like
A B C D E F

If the combos are

A-B-C
A-B-D
A-B-E
A-F

That means 6 hotbar slots are being used for those 4 combos

So you could have a setup like

Hotbar 2=F-D-E
Hotbar 1=A-B-C

However instead of that what they could do is this

You have 4 combos

When I hit A, A now turns into B, when I press B it turns into C and then the combo is done it reverts back to A(or if combo timer runs out)

So now you have this

Hotbar 1=A(B)(C)-A(B)(D)-A(B)(E)-A(F)

So intead of having the hotbar use 6 spaces for 4 combos it now only uses 4.

Space could further be saved if say instead of setting each hotbar for x combo it instead does this

Hotbar 1=A.

Hit A and the following happens

Hotbar 1=B-F

Then if you hit F it goes back to A but if you hit B then this happens

Hotbar 1=C-D-E

So now in combat you would have this

ABC=1 1 1
AF=1 2
ABD=1 1 2
ABE=1 1 3

In that scenario now only 3 spaces are taken up for combos.

So overall there are manners in which abilities could be changed to make it easier to add but for now it is wait and see, we'll most likely know by next Thursday during the live letter, since they do plan on talking about battle system changes, though my bets are on they will do one of the two options listed above so that they can add more abilities without making the hotbar to cluttered.