Monster unlocks should absolutely not be tied to kills. Have only basic monsters on continent 1, introduce new ones on continent 2, and add in elites on continent 3.
As for what to unlock with monster kills -- it could be anything. New enchant types, for example. There's already a precedent in the game: You unlock triple jump enchant by killing 5 utahraptors.
I actually like the monster unlocks as opposed to "unlock via continent" for one simple reason: It didn't take bloody forever to meet the new monsters.
I like the ideas behind it: the game retains/increases challenge as you get used to each enemy type. Once you've killed 100 bats, you should be familiar with their attributes and attack patterns; so now's the time to throw in a new type of bat, which is pretty similar but with a couple of improvements to make it stronger). And it doesnt matter what continent I'm on, it can still happen.
It's true, I could have just RUSHED through the first continent, but I'm not the type to do that, so I spent a good deal of time there. And it was nice to see some new monsters show up so I wasnt JUST fighting the very limited early-game selection of foes that entire time. Before I'd even gotten to the overlord, there were all sorts of crazy things that could appear, including all of the elites. It made things INTERESTING. Particularly the boss unlocks. Having JUST the robot and the blue blob as bosses on the first continent would have been...... very boring.
By the time I got to continent 2, I was familiar with all of them, enemies and bosses, which is good, because continent 2 adds piles of new stuff as it is; dont have to absorb ALL of it all in one huge blob, if you did enough fighting on the first continent.
Also, one thing I thought of: Players might be less inclined to just AVOID killing things now, since the store was popped into place. You get the shards by doing that, so more new players might be willing to brave the new monster unlocks, since it's getting them shards.
