I only post this because I think many people feel the same way as I do about this issue.

As many should know, the Mothership was heavily nerfed for the blizzcon version of SC2. I believe it had a unit cloaking field, a nerfed version of the planet cracker/time bomb, a ground only attack, and most disturbingly, no build restriction.
In that same version, the Star Relic, a sort of arbiter replacement (aerial spellcaster w/ cloaking abilities) was removed in favor of a ground support unit called the stasis orb.

It seems that these changes reflect certain conflicting desires on the part of the developers that haven't quite yet found a balance.

On the one hand, they want the Protoss to have a balanced fleet with the flair and interest that it had in SCBW, and it seems that an arbiter-like unit is a perfect fit. I agree, and I would love an aerial spellcaster with army-buffing abilities (like the cloak field, or, as was suggested on the sclegacy forums, a shield sharing field) The arbiter is one of my favorite units from SCBW, and while I don't necessarily want it to return, I want a unit in the protoss fleet that has as much character as the arbiter did, which is why I was excited about the Star Relic, and disappointed to see it removed.

On the other hand, they are struggling with the mothership, trying to make it balanced but not redundant or boring. I understand that perhaps black hole was IMBA as it was in the WWI, but I think removing its air to air attack altogether is a bit drastic, especially if it means that the build limit must be removed. The build limit was one of my favorite features of the Mothership. It made so much sense flavor-wise, and it really made the Mothership stand out as unique. Personally, I'd rather see the Mothership be almost as powerful as it was at WWI, EVEN if it means making it prohibitively expensive, or nearly unobtainable. While I like the idea of giving the Mothership a cloak field, it's not worth it if it means the Motherhip must be nerfed to the point of being just your regular Every Day unit. The Mothership should be a whole different tier from the Carrier and Arbiter/Star Relic, which is why the Protoss still need an aerial support unit.

The devs also seem to be struggling with the issue of an early game ground support unit to spice up the stalker/zealot doldrums. It's my opinion that the stasis orb was less a direct replacement for the star relic as much as for the Soul Hunter. Many people seem to think that the stasis orb is a temporary shot-in-the-dark at replacing the star relic as support caster, but I think this would be a major mistake, not only for eliminating the aerial support caster role, but for under estimating the need for a dedicated early game support unit. I personally am excited about the stasis orb, but I don't want to see it replace any of the previous units, so much as I'd like to see it replace the Shield Battery as rush support/defense. However unlike the shield battery, I'd like to see the stasis orb become "scalable" in the sense that it can still be effective late game in the right situation.

Turning the stasis orb into a ground based "spellcaster" arbiter replacement would be a dire mistake, as it would encroach upon the High Templar in too many ways. So would throwing it out completely when the arbiter replacement issue is resolved, because the stasis orb has REAL POTENTIAL as as an early game support unit. If the stasis orb is designed carefully enough to be cost effective early game while requiring skillful micro, it will open up many dimensions in the protoss rush. I have many ideas about stasis orb design that I would love to share, though I won't clog this post up further by posting them here.

In summary, Keep the Mothership one-of-a-kind, give us a worthy arbiter replacement, and turn the stasis orb into an essential early game support unit. Don't conflate these tasks, whatever you do.
Nerfed motherships, bland ground units, and an unbalanced protoss fleet are no-nos!