On OpenNos, there has never been any limit to the number of monsters following your character. You can already aggro all the map as long as the monsters detect you.
If you want ALL monsters to be hostile, you can easily change that in the Database by changing the IsHostile boolean to true.
You can do this with a simple query such as :
Code:
UPDATE NpcMonster
SET
IsHostile = '1'
WHERE IsHostile = '0';
This query hasn't been tested but if I remember OpenNos database correctly, this should work.