Yep, totally doable, and in fact IMO a good sandbox should have a couple of metaplots running in the background. The important hings to remember when doing this:
(1) Don't be disappointed if the players choose not to "bite" on the metaplot -- let them follow their preferences;
(2) Resist the urge to railroad them back to the metaplot;
(3) Let the metaplot continue to run to its logical conclusions if the PCs aren't involved, revealing bits of it from time to time. It will make the world seem more realistic and dynamic if you do -- but:
(4) Be careful not to spring the "You're screwed! You ignored the metaplot!" ending on your players, even if that's the *logical* conclusion, since that violates rule
[URL=http://www.enworld.org/forum/usertag.php?do=list&action=hash&hash=2]#2 [/URL] . It's a fine balance between having a realistic world that does its own thing in spite of the PCs while still giving players the illusion of choice.
The key to sandbox-with-metaplot is maintaining the illusion of choice.
This reminds me of my first campaign when I was VERY fresh to the rules and to DMing. Only, I voilated
#4 because it totally was how the game ended. It was actually pretty funny that way.
I had a big sandboxy area for the players to explore, interact with and play around in and then Wham! The party kicked off the adventure of their choice. The followed this up by traveling from one major city to another, never staying anywhere very long and preferring to stay under the radar.
Eventually though, I had the consequences of their unfinished adventures (or jobs) come back and bite them.
For example, they eventually decided to settle down only to be attacked by a thieves guild who they had wronged earlier in the campaign. The city was attacked by forces from another city, declaring war and whatnot, due to provocations a PC had done in said city. It was made funnier by that character no longer being around (or in the party, I forget which) at the time.
Hell, the entire campaign ended with them finally completing the final mission of their first quest after everything had gone all wrong. Only to have them mess up the instructions they were given concerning said quest and releasing the big bad. In a dialogued cutscene I described how the BBEG (in this case G represents Gas) consumed them and started to spread and eat its way across the world. Not only did they screw up but they ended the whole world in screwing up. It was a very fun game and stories are told years later because:
- I was willing to let my party pick what they wanted to do. I only decided on a "metaplot" after they picked the adventure in the first place.
- I was willing to let their actions have real consequences, good and ill. Their reputation followed them from two different cities, they were able to start war, lay down roots and by poorly managed actions even end the world.
- I had a lot of interesting places to explore. Some were preconceived but a lot was just winged when they got there, adding elements or subtracting them as needed and as the party used them.
In general all three points I listed here would help in the metaplot-sandbox game you are talking about. Railroading is necessary to a certain extent if you want to have consequences, a plot or anything "meta" about the game. But just make sure they doing see the rails and everything is fine. Make the PCs important and allow them to be the movers and shakers, or at least the ones with the major changes. If it happens that they ignore the plot they start that is fine, just have the effects catch up to them. If you've ever played RPG video games you'll get what I mean. They can do all the side jobs they want, but real plot advancement only occurs when they deal with X or Y. If they fail to deal with X or Y, or take too long, have Z happen instead. It adds excitement and helps things move along - a solution that video games rarely (if ever) consider.
Hope this helps, quite possibly doesn't.