Previously Turtle and Gqldn have explored this topic and have made suggestions for it, this post intends to expand those further in terms of implementation.

This siege region should to be more of a KOTH or CTF type gamemode with only vanilla pvp involved.

What it should work like -

Teams will fight for banner control at the center of the Siege region and the team with most points at the end will come out victorious, points will be awarded for kills and banner control like normal, these banner control timers will be shorter than the normal than the usual 7 mins to make this a fast pace 30 min max combat situation. Some custom structured banner can be made maybe so its like a nexus or statue from league which the players are fighting to control? would make it look bit fancy I guess.

There will be keep inventory on in this region when its being sieged so players can come back after dying but they will still get a bit of armor durability chipped off, This region would also be world guarded to stop players for making traps and ditches beforehand.

There can only be a limited amount of players involved in this region, lets say 10 per nation, this team would be created by the party system and only the 10 players first added in that party can partake in this region for that nation. Allied nations can go against each other to simulate the same feel as fighting allies in an movecraft related region.

When a party intends to fight for a siege region they have to be standing in the nations capital, the party leader will then run a command like /warp siege regionname for a party warp before the region starts, the players are teleported the command will cycle every 5-10 mins and anyone who dies in the region will get teleported back to it when the timer hits. the players that are alive and already in the region wont get teleported around.

Once a nation gains control of this region they gain siegewar artefacts or something similar as towny resources and money ofc.

There should be a limiter or something involved that stops every nation to just partake in this region and make it a mosh pit. This could maybe be some sorta token or coin thats earned by doing offensive sieges on nations and winning them and then that coin/token can be cashed out in the pvp region or it should take some money from the nation that intends to fight for it. lets say the region pays out 7k for holding it then it should take at least 3k to siege it.

why a warp command and not on foot travel or some other movecraft related travelling? Because its a bit hard to just go to a vanilla region with movecraft and expect to just leave it in the wilderness and also because players cant just go back and forth with it fast enough after they die. It also means that towns wont be made right outside of the region for the sake of getting there faster and even if they are made it wont offer them anything better as players are still required to party warp for the region to be registered for it.

Pros:

Adds in a fresh new mechanic a new siege region.
Gives vanilla pvpers a region to partake outside of just nation sieges.
Good towny resources wont be limited to just researching for them and can be granted by controlling this region.
Towny artifacts can then somehow be involved in siege regions as an overall, the top 3 nations that controlled the most regions for a month will get awarded with them, this forces a nation to have both good movecraft and pvp force backing them for these regions and adds in another depth and not just a money farm reason.

Cons:

Seems like it will easily become a mosh pit of multiple nations running around and might need a banner code to count multiple nations points instead of just the 2 going back and forth.
Requires bit of balancing and spawn selections so the teams aren't just spawned right next to each other or something.
If allied nations third party then some other measures might have to be taken to fix this further. Prolly stop allied nations from joining the region all together or giving allies option to join another nations party and warp for them? needs to be ironed out.

Location where this region can be created:
Central Australia or Russia offer the best locations for these as they are empty and have big patches of land thats mostly flat.