GM advice needed: where should I go with this game

I'm currently running an real life game for a bunch of new players, and while I sorta know where I want to keep going, I don't love the idea I have so far, and would love some feedback. Hopefully the one player I know is on GP doesn't actually read the forums :p

So the game as it stands, started at lvl 1. First game, the players found themselves fighting goblins led by an orc warlock who had a mysterious orb on him (which the players took). As they continued, they found two more orbs, finally coming across a mysterious tower deep underground (which obviously didn't belong there), which the orbs reacted to. They made their way to the top of the tower, where we stopped the game for next time.

So where am I going with this? I've decided to do a game around a beholder treasure. I plan on having 10 orbs, which placed in some location along with some mcguffin (spell, item, something) held by the mage at the top of the tower, would release said treasure. So far, the idea seems good, plus the twist: But now where do I go? 10 orbs with 3 recovered leaves at least 7 adventures open, as not every adventure needs to result in an orb. My first thought was have the mage at the top of the tower hold another 3, and have the characters wander the caves towards the underdark for a few levels (they're 4 now), and end the game around 10. Or take it out into the wild and see where things go.

Anyone have any thoughts on the overall idea or any sub ideas I can throw in to tie it together more? Any ideas on how to make the individual quests between the game more fun while relating to the overall idea? Or should I just say screw it and drive them in a direction?

I would say you need to decide why the 10 orbs were separated.

Were they intentionally hidden by the treasure owner? as an initiation test, maybe? a rite of passage that someone had to go through to prove they were worthy of the treasure? [they want a worthwhile hero to find the treasure]

Or was the treasure a cursed thing, and a council of wizards scattered the orbs so they would never be found. [they want nobody to find the treasure]

Was it an accidental thing, the result of magic gone wrong? (like losing the code to a safe)? [they want to regain access to the treasure themselves]

Are the orbs alive, and have scattered by themselves? Maybe they produce retractile little legs and roam... to make things trickier, the magical beasts know teleportation spells. [catch the critters]

Thinking about this might inform where the orbs ended, if there are clues in books, prophecies, legends...

Then for variety, each orb/group of orbs could be hidden in wildly different places.

-A gigantic beast swallowed one. Heroes need to be eaten to recover it
-one is hidden underwater, they need spells or other means of breathing underwater
-a monstruous bird believes one of the orbs is its egg and keeps it in its nest at great heights
-one is frozen in the ice...
-one is used as a crystal ball by an enchantress

You could have the original tower master have created an organization, let's call it BETA, giving the 10 items to 10 lieutenants, each in charge of their own branch of the organization. Much of BETA has since collapsed and some orbs fell into other's hands or were lost. Have a couple lieutenants or their heirs have several orbs and the rest scattered.
You could have some orbs in the hand of our watched over by an enemy of BETA, called DELTA, and they could conflict with the party who is trying to collect the orbs. DELTA may mistake the party for working for BETA, which might actually be true. If this gets cleared up DELTA world still oppose collecting the orbs as they have suspicions of the orbs true nature. Different branches of BETA may aid or oppose the party as some do not with the orbs collected and the mastery plan activated. . .

I agree with Dr. B, in that deciding how and why they were separated is of utmost importance. Without knowing that, you're just flailing about in the dark. Maybe the orbs are the ten eyes of the beholder (magically changed) and bringing them together is what causes it to awaken. The mage knows this and sends agents out to retrieve them before the characters do. This would give you a party of counter agents that are working against the PCs.