Scratch is a downloadable game creator you can use to possibly make an arcade for your site. But I was thinking, you could make more than just a game for your site, what about a town?
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In this town your member can explore, and if you know how you can make areas (Kinda like grass in Poke'mon) were after walking around for a while you encounter a wild -insert pets name here- you could battle it and after the battle you would recieve a promo code for that adoptable, you can enter it and your member will recieve that adoptable.
With the town you can get adoptables the same way, you could create pet shops where you can click an adopt, be asked if you want it, then a promo code appears. I think its pretty neat you can do that with Scratch and the Mas.
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-I will be posting some tutorials here soon on how to make this.-