If you mean "big trade" in the sense of "for a big name now," then no, I think that's the last thing the Spurs are going to do. On the contrary, I believe the Spurs are in anything but a "win now" mode. I think the "after Duncan" team is already built though not yet unveiled, and the true value of AB is going to be the draft picks he allows the Spurs to trade for. Signing Bayne appears to have nothing to do with winning now, and that's what seems "off."
******
Keep in mind that the NBA is a business, and team owners don't own teams as a philanthropic service to the community. They own teams because those teams are supposed to make them money, which means asses in the seats, which means putting a compe ive product on the floor night in and night out. Coaches are coaches, and as much as we may like to give the coach "CIA Pop" credit, keep in mind his paycheck is signed (or more likely, stamped) by one Peter Holt. Peter Holt runs this, because he is the one to profit from this. Holt is also a highly successful businessman; you probably pass by his company (or one of their vehicles) every day on your commute to work.
The cliche as a business owner is that you are supposed to "stay two steps ahead of the compe ion," but in practical terms that means that your job as CEO, president, whatever, is to be simulating what the compe ion will look like years from now and preparing your current product to meet and exceed those future demands. A decade ago, when Duncan, Parker, Manu, Pop slowly started to show that they alone were enough to create a compe ive team (which means asses in the seats, which means money in Holt's pocket), Holt had to start planning for the future. And because he's a smart businessman -- as you can tell by the successful company he runs -- he was able to delegate the appropriate future-orientated tasks to the appropriate personnel.
*****
A decade ago Holt knew he didn't have the deep pockets to compete (and thus make money) against most of the NBA in the NBA free agent market, so he started going over seas where all the sudden his small pocket book by NBA standards still made him a big fish in a small pond there. In business-language, it's called the "blue ocean strategy" -- you move where others aren't. Parker, Ginobili, Splitter, Beno Udrih, De Colo, Mahinmi, Oberto came from that. Countless more could have, but those that did are still a solid haul that proves the strategy worthwhile.
In regard to domestic talent, where the exchange rates weren't as good for Holt, he decided instead to (military metaphor here) "snipe" (sign) the "high value targets" (role players w specific skill set) which furthered his mission (making money by staying compe ive) -- i.e., the FA's who could be bought cheap, who each individually had one or two superior traits, and who combined became the much cheaper equivalent of what teams in larger markets could afford to sign. Steven Jackson, Jarren Jackson, Steve Kerr, Danny Ferry, Robert Horry, Gary Neal, Kevin Willis, Roger Mason, Barry, Van Exel, terry porter, samaki walker, antonio daniels, mario elie, jerome kersey. Either big names past their prime where they still have one or two major skills left, or budding names with one or two skills upon which they hope to launch a career. And for players on both extremes, the Spurs were either the perfect launching pad or the perfect landing pad for their careers. What benefited them, also benefited the Spurs, which benefits the fans, the abundance of whom financially benefited Holt.
It's all about competing, because for Holt, it's all about making money. Like I said before, owning the Spurs isn't some philanthropic community endeavor for him.
*******
So now in that light, look at the potential of the Bayne signing -- it looks like the final piece if you're playing a half decade ahead like Holt is. De Colo at PG, Green/Neal at SG, Kawhi at SF, Bayne at PF, Splitter at C. Take their current projections, add in expected "changing of the guard" in the league, and go out about 5 years. In 5 years, that's a very compe ive starting 5. It might not be a championship team, but it's certainly going to be compe ive enough to guarantee asses in the seats which in turn guarantees Holt that he won't have to think of selling the team (which, 5 years out, is something you should probably start working on) because it isn't profitable to him.
*****
Now about that "bigger trade" you mentioned. If Bayne fills the remaining spot in the future starting 5, then it's safe to say now that the Spurs could use some draft picks. Unless those picks' last name is "Leonard," it will probably take them a few years anyways to grow into the Spurs system. Bonner, Blair, Jackson, Green/Neal, and to a certain extent Parker or Ginobili is going to look like some awfully tempting trade bait for a lottery-bound team in "win now" mode because their owner wants to guarantee asses in the seats (and money in his/her pockets). Millionaires and billionaires only got that way because they were smart with their finances to begin with (or their name is Mikhail Prokhorov, and they simply own all of their native country), in that light EVERY team is in "win now" mode because they need to put asses in the seats now in order to make a profit now.
Tim Duncan's REAL value to the Spurs is that he guaranteed at least a decade of Spurs compe iveness, high attendance rates, and thus profitability for Holt. Championships were a bonus; the real reward for us fans was the holding pattern of profitability Duncan provided for Holt. It allowed him to think, and plan, for the future.
*****
Bayne might not turn out to be much of anything at all individually, but that "off" feeling I have with his signing eminates not from the signing itself, but from the fact that it is a manifest indicator that Holt isn't working in "win in 2013" mode; rather, it feels "off" because it appears more like Holt is signing players now in order to win in 2023.