I'm planning a project around and west of Saltpans in the Riverlands, across the river from the God's Eye Mega. Some of our terraforming plans would be beneficial if applied to the south bank, which would slightly affect your megaproject. This post references the extant terraforming plans for the following regions of the megaproject: Harrenhal's eastern reaches, toward the border with Darry and Whitewalls; Whitewalls itself, from the border with Darry and Maidenpool to the God's Eye itself.
The relevant portion of my own plans concerns the Trident and its terraforming. I plan on implementing fluvial terracing as the primary elevation feature, where the lowest terrace is the floodplain itself, composed largely of alluvial plains and fans, and containing the bulk of the riverside agriculture. This will naturally lead to a significant rise in elevation (est. 10-20 blocks above sea level for every 100 blocks from the riverbank) as the drylands approach the foothills of the mountains to the north.
I believe it would be beneficial to implement the accompanying landforms along the south bank of the Trident, tapering off into deeper fens and bogs as the river empties into the Bay of Crabs. These plans directly affect the following projects: Darry; Lord Harroway's Town; Maidenpool; and on the northern bank: Saltpans; Hawick; Crossroads Inn. Care will be taken to coordinate with the leads and future leads of those projects, as none fall under your purview.
What does affect your megaproject is the final elevation change inland of the terracing on the south bank. Fluvial terraces can be asymmetrical relative to the cross-section of the river, and the north bank also has the foothills of the Mountains of the Moon to contend with, so the exact elevation increase is subject to change (est. 10-20 blocks ASL total).
Per the Whitewalls section in the Mega Application; "The further away from the Gods Eye the more wooded and hilly... [the terrain becomes]". This plan is rather vague and should still be easily implemented atop and within the elevation increase caused by the fluvial terracing closer to the riverbank. The terrain would then drop somewhat as it approaches the lakeshore, back down to sea level at y:34.
Harrenhal and Harrentown make no mention of terrain and terraforming, so I can't predict the effect on those locales. I do find it unlikely for the castle to sit entirely within a floodplain, at least without a moat to redirect the overflowing lake into. The most realistic solution would be to raise Harrenhal and Harrentown onto a higher terrace, but that is a big ask. Otherwise, the terrace could dip around Harrenhal specifically, around the east and north towards Lord Harroway's Town, and the castle could see an extension of the moat around Jake's.
All told, my requests for terraforming within your Megaproject are relatively minor and are able to accommodate your existing plans with some adjustments. I want to have fluvial terraces along the south bank of the Trident, within the projects of Maidenpool and Darry, which will climb to a highpoint. All the mega would have to do is match the highpoint. From there, any terraforming decisions would be under your purview, though I'd love to see some complex geomorphology involving the lakeshore! Perhaps more fluvial terracing, or some alluvial plains, approaching the shore and the fens I believe are in place already.