The border-region problem is roughly this:
Synchronization of projectile/players/items/monster leaving region A places undue burden on region B(and Possibly C/D if on the crossing of region).
Any events such as combat/area-of-effect spells are effectively duplicated.
A.Ignore data from other region server. Place the burden on player to appear on both servers: lowers client performance, may cause desync and errors. Low impact on server.

B.Send data between servers to sync projectiles and player locations, within a certain distance from border. Looks best, with high resource use and latency increase.
Limiting what is shared is the key.

C.Regions don't share borders(no seamless open-world), instead access is via portal or instance hub. Realistic and least resource intensive.
Just make regions self-contained mini-worlds disconnected from each other.