There are infinite ways.
The easiest way to approach this is to ask your players themselves: what sorts of things would you like your characters to accomplish? Other options involve you, as GM, simply dropping a plot onto them-- once they have come into possession of the Death Star plans, through whatever means, they're enmeshed in the rebel plot no matter what they do.
You can draw on your own ideas and knowledge of the characters for motivations: once someone is fabulously rich, or at least not indebted enough to be forced to work, what might they do? What might they care about?
I wouldn't pull back any of the things they've already purchased (something like that would seriously displease me as a player), but the fact that they've had a couple of big scores doesn't mean that they're free and clear of all difficulty or desire. A lot of that is in your hands as the GM.
Debt is a fluid concept, and you can still use it as a driving plot element. Even if the casino heist goes off exactly as your players have planned it, they'll have stolen a lot of money from someone sufficiently connected and dangerous to respond. They may have X credits in their pockets, but the casino owner may very well consider that to be a personal debt of X credits owed to him- or herself. Or maybe the money they stole is "hot"-- counterfeited, part of a money laundering scheme, etc., and so even though they have it they can't really spend it (safely). Or the only way to make it spendable is to launder it (at an unfavorable rate to the players), or plan some other scheme.
Maybe the audacity of the heist is much more of an insult than the value of the cash itself, and so players can't simply pay the casino owner off-- they'll do jobs for the casino owner until he or she feels whole, or else they'll face endless waves of assassins (or other inconveniences) which indefinitely prevent them from doing more than surviving (at best).
Perhaps the money is only a peripheral motivation, and the characters really want the fame that comes with being a famous band of smugglers. In that case what they want are more and harder heists to pull or blockades to run. The money's part of the offer for future jobs, but it's the challenge that's enticing.
Or maybe they didn't smuggle quite as well as they'd thought, and the authorities are on to them, hunting them to dissuade others from the (apparently very easy-to-come-by) rewards from smuggling.
Maybe it's all of those!
The most important thing: a motivation that can be satisfied in a single session is not a good driver for a plot.
Dice confound all GMs at some time or another, but in planning out this game and running this session it might have been better for it to be impossible for the players to clear their debts with a single job available to them. The mismatch is the problem. They should have owed far more money, or their smuggling opportunity should have had a much lower ceiling on how much money it was worth.
Future motivators should have that same consideration in mind. Whatever goal you end up assigning them (or that they assign to themselves), their missions should be fundamentally incapable of satisfying those motivations so quickly and easily, luck or not. I hate using GM powers to enforce that, but sometimes it's inevitable. Players come to play the game, not so much to win it.