Even after leaving nostalgic feelings aside, as you suggested, though not viewing it a very wise of a premise, there are a number of potential issues that I see in the aforementioned change:
Starting off, we are supposed to discuss offensive, very offensive, effects, because the Army of the Dead does occupy such position in the whole scheme. On the contrary, the sheer idea of summoning the White Tree and focusing on the New King's later reign, look like a perfect foundation for a defensive/long-term spell, which is not our case (which should not be our case, I mean). The attempt to incorporate Oathbreakers into the main body of the concept, neither adds value to it, nor does it really resolve the underlying problem: we need an effective (game-changing, hopefully) summoning, that functions in the simplest way possible (bear in mind that time is already a quite impairing of a constraint). Having to cast the spell on whatsoever building, just to make use of ghosts in battle, would result in being utterly unnecessary and counter-productive (given that the phantoms' own timer does its limiting work splendidly, already).
I also advise against basing an ultimate spell on a single hero, however great or preponderant he may be, considering how Aragorn's major narrative arc is marvellously explored via his very abilities, of which his crowned self is currently a vital aspect (the last stage, effectively). Why should we stress his royal character even further? Would that not make him less unique as a single hero, if a final spell were to reiterate said qualities?
There might be another lore remark to bring to the public's attention: being the faction centred on the War of the Ring, primarily, I guess it would be improper of us to concentrate too much on peace-time events (his subsequent reign as the king of the Reunited Realms, I mean). I find some contradictions here, in spite of the fact that one may play with King Elessar, now, without the need of grander features that remind of him again.