Do an event based monster enounter system for the world map.
Nightwolf did suggest that, but did not tell you how.
Now do this... suppose there are 7 different kinds of encounters on that square... make it choose a variable between 1 and 20
if it lands on 1, 3, 6, 10, 12, 15, or 18 it's a monster battle. otherwise nothing happens. or say do 1-7. it's a battle for each of those.. but
on 1 make it roll a variable form 1-3, if its 1 the battle occurs, for say 7 make it 1-10, if it land son 1 the battle occurs. Now the way this way works is, some battles occur more than others. Another method of doing this is rollign one variable..
Roll the variable from 1-120 if it's 1-7 it's battle number one 10-16 number 2, 20-25 number 3, 30-34 number 4, 40-43 number 5, 50-52 number 6, 60-61 number 7. All others nothing happens.
Although I'm pretty sure RMXP doesn't use a world map system