I would rather a smaller scale Minas Tirith, like this https://ideas.lego.com/projects/36275, than just a minifig-scale gate.
My wave three ideas:
The Battle of Pelennor Fields - $129.99 (similar in size to Helm's Deep)
Minas Tirith main gate, 2 outer wall segments, the Citadel, the White Tree, 2 inner wall segments (The build focuses on the gate, the Citadel, and the 2 inner wall segments.)
Fellbeast, Witchking, Shadowfax, Gandalf the White, Gothmog, Denethor, Faramir, Gondor Pippin, 2x Mordor Orc
Gondor Soldier Army - $29.99
Outer wall segment with trebuchet (Adds on to Minas Tirith. This sets focuses on expanding the outer wall.)
3x Gondorian Soldier, Horse, 1x Rider of Rohan, 2x Armored Mordor Orc
Mordor Orc Army - $39.99
Siege tower, trebuchet
Armored Mordor Troll, 4x Armored Mordor Orc, Horse, 1x Rider of Rohan, 1x Rohirrim
(The Real) Witch King Battle - $14.99
Scenery (grass and rocks)
Witch King (Helmet + hood. Army build the 9 Nazgul), Eowyn, Rohan Merry
My wave three idea focuses on the Battle of Pelenor Fields and army-building. The more Gondor Soldier armies, the larger your Minas Tirith defense. The more Mordor Orc Armies, the larger your opposing orc army. The more Witch-King Battles, the more Nazgul. The idea is that while the sets are initially small, the more you buy the larger they become.
I included the Riders of Rohan in both army sets to appeal not only to those army-building Gondorians and Mordor Orcs, but also to those still trying to army-build Rohirrim.