Try not to have reqs for reqs' sake alone. For instance, don't care about the leaderboard climb? That makes trophies pretty much meaningless!
The important resources for guild development are gold and seals. I'll go with seals first, since they're simpler:
The two thresholds for getting benefits from seals are at 20,000 and 40,000. Assuming your guild has 25 active players, reqs of 800 seals per week should hit 20,000 easily, especially since more dedicated players will try to hit 1500 for the extra guild keys for themselves. It's pretty tough to hit 40,000 without 1500 minimums, but one compromise for that is to have a dedicated week with the seals req being 1500, either once a month or once every 2 months; that way people who really want level 6 chests can save their seals and keys for that week. Then again, people who care about that level of optimization are probably already in a hardcore guild
Gold! Gold is used for statues and bonuses. 500k reqs is enough to get max level statues and full bonuses each week. 250k will give you a strong chance of getting 2 or 3 of the six bonuses, particularly if your guild is smart about donating. 100k should get you a statue maxed with smart donating. No matter what, you'll get a decent amount of resources still since all tasks at low levels are so cheap.
The final aspect is enforcement: You can let mooches hang on for a week or two or you can deal with them immediately. Having screenshots on the ends of weeks can help you see who hasn't made reqs over a period of time. Some members will prefer a forgiving guild leader, others will see it as a drain and move on. The choice is yours!