Replacing wildlife is a relatively simple procedure, but it might be a bit confusing for someone new to modding and it can cause some unfortunate consequences if you don't pay attention.
As for sieges/megabeasts, I'm afraid that setting specific values is a little beyond the controls provided, or at least my knowledge in working them.
However, I think I have a suitable replacement for you.
Go into the Dwarf Fortress/raw/objects folder, and find the text file labeled "entity_default". Look inside it until you find the goblin profile. It's a massive block of text that you really don't need to pay attention to except for a couple tags near the bottom. You are looking for "[BABYSNATCHER]" and "[AMBUSHER]". Delete the tags by simply removing the text, and you've got yourself some fun times ahead.
What this does, essentially, is prevent goblins from sending child-snatching thieves or ambush parties. This leaves them only one method of interacting with you: sieging.
You can get sieges as early as the end of the first year, and they will just keep coming. Doesn't matter how much your fort is worth or how many dwarves live in it, the goblins will siege.
You may find yourself slightly bogged-down with goblin junk, however. If you look closer to the top of the goblin entry, you'll see a lot of stuff that looks like it's talking about various weapons, armor, clothes etc. Well, it is.
Find every tag that relates specifically to clothing ("[ITEM:ITEM_HELMET_CAP:COMMON]", or something like that) and nuke it. You now have naked (except for the weapons and armor, which you are going to leave in, right?) goblins that will attack you on a regular basis.
Hope that helps.