I'm going to have to completely disagree with you here, there is no limit to the sidequests apart from the time they have to put into them, you simply have to use your imagination to come up with compelling reasons to do them.

First thing first, make the combat much harder then it currently is, this instantly gives you reason to not only seak out as many heart pieces as you can, but also provides incentive to look for additional abilities that may help you in combat. These can take any form you want, be they arrows, spells (ala AoL's awesome spell system), sword techniques, additional weapons of diffrent types, MM style mask abilities (which I assure you are used for far more then simply getting more heart pieces) or whatever your imagination can cook up. You don't even need to limit the game to 20 hearts (which was a number that was only introduced in OoT), why not have 30?

Furthermore there is no reason why auxiliary goals cannot be introduced. MM already touched on this in several ways, as the sidequests in that game allow you to:

- Discover more about the world and the people who live in it
- Unlock alternate ending sequences
- Gain awe inducing abilities/forms

There is no reason as to why this cannot be expanded apon further. Give me secret optional bosses which are much more difficult then anything else, access to hidden areas/dungeons etc.

Remember that sidequests don't have to simply be things that help you beat the main quest, they can be goals within themselves.

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"And for a time it was good"