A starting point -> the world?
It's always a good practice when building a system to decide its boundaries. What is the scope of the system? What will be in and out? Will you want it to handle your professional life? Your personal life's issues also? Your family? The world's poverty? Do you want it to manage the weather? Do you want it to handle your finances? Your dreams? Your recurrent tasks? Your children's future? Your mother's IRS?... Where do you draw the line?
This theme has been the source of many books of wisdom. You've probably heard it briefly summed up in some saying similar to: "I hope I have the strength to change what I can change, the humbleness to accept what I can't change, and the wisdom to know the difference".
To this, I would add the "decision" part. There is some part of the world that you can change, but you are free to decide not to. That is, in fact, of major importance in building your system in a manageable way. The system should be quite robust, in order to cope with the handling of a big number of opportunities, tasks, projects, and all, but when you are not clear about what you want to keep in or out, don't expect the system to decide that for you. And if you don't draw the line, you just might end up with an unclear and unmanageable giant collection of information, not really that action-oriented, thus not that useful, because it might not really represent issues you want to do something about.
Commitment management
So how do you decide if something belongs to your system or not? I would suggest you base your decision upon the commitment concept. In a simple way, a commitment is something you promised and intend to fulfill.
You can have commitments to others or to yourself (in terms of your system, that's basically the same). Some of those commitments are implicit, others are more explicit. For example, you may have an explicit commitment to your bank in terms of your mortgage payment every month, and probably have an implicit commitment to yourself in terms of being happy and having fun in your life, or the implicit commitment of being present and support your children.
Any of these commitments, if not handled properly (whether it's failing the bank payment, or failing to be present to your kids), will disturb you inside, endangering your joy and self-esteem, no matter how "successful" you may appear to be. That's why it is so important to clear some of these. Unclear commitments make for unclear notion of fulfillment. You may have an implicit commitment of visiting your mother once in a while and not know if you are visiting her enough times... so you'll have some inside disturbance from that.
Writing down (or at least mentally clearing) "mission statements" for each of the multiple roles we play in life, can help a lot. I'll be back to this later on. For now, bare in mind your system will ultimately be helping you to manage your commitments and enable you to enjoy each moment, knowing everything is being handled properly, or, if it's not, orient you towards whatever needs to get done in order to regain that sense of balance and direction.
19 | SYSTEM'S SCOPE - COMMITMENTS