For what it's worth, here's my list of favorite and plausible free agent targets. I'm only including players that I think the Spurs have a realistic chance of getting. Iguodala, David West, Tyreke Evans, and Kirilenko would be great additions but are just not likely.
1. Brandan Wright C/PF
Efficient scorer and shot blocker, long and lanky defender. I think he'd be a perfect complement to Splitter. Splitter would play against the bigger, more traditional centers and Wright would play against faster small ball line ups. Duncan And Wright side by side would be the most efficient and disciplined shot blocking tandem in the league. Would probably demand at least the MLE.
2. Mike Dunleavy SG/SF
Reminds me of Brent Barry. Very good 3 pt shooter, high BBIQ... not a great defender, but has good size and length for his position(s). Most importantly, would not be expensive. He accepted 3.7 mil/year in Milwaukee and would most likely accept the same or less from the Spurs.
3. Shaun Livingston PG/SG
Low turnover PG with good floor vision. Doesn't do anything spectacular, but doesn't make many mistakes either. Good passer, plays defense, not much of an outside shooter. With Manu clearly in decline, I think the Spurs need a reliable ball handler to back up Parker. Again, won't be very expensive.
4. Al Farouq Aminu SG/SF/PF
Incredible rebounder, and his FG% has increased in each of his first 3 years. Young and athletic. I don't understand why New Orleans let him become an unrestricted free agent. Aminu, Leonard, and Duncan on the floor together would be one of the best rebounding trios in the league. At the right price, he'd be a great addition to pair with Leonard and Green for the next few years.