In my opinion, it depends on how you set the recipe systems.
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I think maybe there are many ways to make a combined item formula.
In my opinion, a combined item can also be made in this way.
Here is one of my thoughts on combined items formulas, you kill a monster which gives you a chance to get items (just like in any online monster farming games, can be a skill book, equipment, etc.). Then, you upgrade them by using your certain amount of power points. (In my opinion, a recipe no needs to be just combined by using at least two items and one recipe.)
If you combine an already upgraded item with a non-upgrading item by mixing them with a gem or a recipe (or maybe they are not needed), then the result will be a much more stronger version of combined item. If you combine two non-upgrading items, then the result will be a weak combined item.
Maybe it is quiet complicated but it should bring in a more challenging game-play. (My imagination is that if DotA includes this feature, then... As a DotA player, you not only need to think of enemies' locations, types of skills and items but you also need to think of their upgraded skills, upgraded items, upgraded combined items or even more of them. I can see that there are no dropped items when a monster is killed in DotA. What's the point for you to kill monsters? Just for golds? Nothing more specials?)
You can even combine any item with an enhanced gem to add more special attributes to it. More of them...!
That is one part on what I think. It depends...