So my thoughts on Shadow Form…
First off, the name itself has “form” in it meaning you are becoming a form of shadow.
I see it just like your forms you shift into as a Druid. I really liked it when your primary spell bar changes to an entirely new bar just like what happens to Druid forms and what used to happen for Warrior stances which is a shame it’s no longer the case.
I like the shifting forms affecting spell bars as you can incorporate that into macros that are governed by your “form” or “stance” which gave you increased flexibility in how you choose to play as you can also just flat out ignore it altogether if it either didn’t matter to you or made things confusing.
On the matter that Shadowform is required to use Devouring Plague is very odd to me as it can be disrupting to gameplay if you are not aware or expecting it yet I suppose it fills that “form” idea a bit more.
On the matter of casting Holy spells in Shadowform I find to be just silly and goes against that entire dynamic of holy and shadow are opposing forces and to imply that you need to choose one or the other to use as they are diametrically opposing forces.
Except as Discipline, you have enough discipline to balance the two forces in shifting from one to the other as part of your entire gameplay. Where as Shadowform you should sacrifice your ability to tap into holy magic to fully embrace shadow magic and be engulfed or consumed by it.
The idea of casting holy spells while in shadow form just dilutes the potency of the impact that those spells make as in the past they were really strong because you had to sacrifice your damage and defenses to cast them so they had to be worth it where as now, you can cast them all you like but they are basically a waste of a button press because of how inconsequential they have become.
Back when you can use holy spells and it pulled you out of shadow, yes it would drain you mana but that was only because it was impactful as the result which gave this ebb and flow dynamic outside of just using DPS buttons as it still allowed you to behave as a priest, it at the cost of your shadow damage and other defensives.
Not too dissimilar to feral druids where you can use your healing at any time but it can force you out of form but because of that cost and risk, your heals can still be impactful. Then you can also get procs to use your healing spells without dropping form to gain both the impactful outcome of healing while not negatively impacting gameplay from shifting you out of form.
So my stance on Shadowform is to make it more part of your gameplay that clearly draws a line between you being a shadow priest and just a priest in general.
This is why my idea from my post way in the beginning is me suggesting that while in shadowform, you could get a proc like effect to deal your next mind spell damage instantly as that would both be very impactful, provide reason to be in shadowform and add a bit of mobility that we lack. This would also enable a more sacrifice to not be in shadowform which means you could put more power into shadows ability to off heal as you should be able to do as a Priest.
But I’m also open to ideas and suggestions on what else shadowform could do or incorporate in order to really flush out the meaning and purpose and give you a dang good reason to make sure you are in shadowform when you are wanting to be popping off.
Regarding the cosmetics aspect, I vote for either the glyph system or the barber shop treatment that both druids and warlocks have enabled to select what they want for appearances.
But I would rather Shadowform stay as an active swapping into and out of form mechanic and avoid being removed outright like death knight presences or watered down like warriors stances.