TL;DR: As players help levelling up nodes, they will increase their node-helper levels which provide permanent bonuses for their characters. These bonuses could be reduced costs for purchasing housing, upgrades on freeholds and reduced monthly upkeep, for example.
Premise
Losing a home node isn’t going to be fun for most players, especially not if you’ve spent months helping to build it up to be your dream node with your dream house. I feel confident in postulating that some people might quit the game over it in the heat of the moment, especially without mitigating circumstances. I don’t want to see disillusioned guild members leave the game, and in general MMOs live and die by the activity of the player base and their social bonds. The goal of this suggestion is mainly to mitigate the feeling of loss after a node is destroyed and increasing player retention in the aftermath.
Core idea
By giving the players permanent bonuses for helping nodes grow, I think the sting of loss will be mitigated, because they won’t be standing there with absolutely nothing to show for the potentially hundreds of hours of work they’ve put into levelling a node, after a successful siege takes it down.
The idea is to introduce one more parallel, or horizontal if you prefer, levelling system to the game that keeps track of the character’s overall contribution to node growth. The higher the level, the better the bonuses.
The bonuses could be reduced costs for purchasing housing, upgrades on freeholds and reduced monthly upkeep. Things that will help reduce the cost of rebuilding their home and freehold in a new node, as well as provide long term benefits. Maybe even give access to a few snazzy cosmetics, like furniture or a building skin or something at the higher levels. And titles of course.
Pretty much everything that contributes to node growth should give node-helper XP, including but not limited to killing mobs in the area, gathering and crafting, donating materials for node-buildings, and doing those node-quests that were mentioned for becoming mayor of divine nodes, that contribute to the node in general.
Citizenship of a node should act as a multiplier for the node-helper XP gained in the area, so players are heavily incentivized to put most of their efforts into their home node. Double or triple node-helper XP for example. In other words, helping a random node you pass by will be giving little node-helper XP in comparison, but not nothing.
Other thoughts
This idea isn’t meant to stand alone of course, but in combination with other things we know - like how the freeholds will be bundled up in a blueprint after they are destroyed, and how all the furniture and such will be mailed to you after a successful siege - I think my idea will reduce the sting of loss a little, because rebuilding your home will be a bit easier/cheaper as time goes on. Specifically for the freehold buildings, the cost reduction could translate into less materials to rebuild your tier 3 smelter, for example.
I also really love parallel levelling systems, and never-ending growth of my characters. Even 5-10 years down the line, I don’t want to feel like the only way I can progress my characters are through better gear or waiting for an expansion that can increase my adventuring level cap.
I’m not dead set on the name “node-helper” or anything, but it gets the point across.

This doesn't need to be in the game at release, but if they decide to add it later on, I hope they at least prepare for it so the node-helper XP can be added retroactively.