Before UFC 102 the answer was simple, Dan Henderson was next in line to face Anderson Silva. Henderson earned his second shot at the Middleweight title by knocking out Micheal Bisping in the second round of their match at UFC 100. The win was Henderson's third straight since losing to Silva at UFC 82.
After his 21 second knockout victory over Demian Maia, Nate Marquardt has been mentioned as a possible contender cutting in line ahead of Henderson. Marquardt has gone 4-1 since losing to Anderson Silva at UFC 73. His only loss in that span was a controversial split decision loss to Thales Leites in which he was deducted two points but still won on one of the judges scorecards.
Of these two I think Henderson should get the next shot at Anderson Silva since it was promised to him first. Marquardt is a great fighter though who has improved greatly since losing to Silva the first time and should easily be in line to face the winner.
The only problem is Anderson Silva wants Henderson to fight Marquardt to see who he faces next at middleweight while Silva takes another fight at light heavyweight.
If Silva gets his wish the only logical choice is to have Silva fight Lyoto Machida. This would be better than the scheduled match between Machida and Shogun Rua. Silva has said though he has no intention of fighting Machida. And Silva would have a less than two months to prepare for the fight.
The only other option at 205 that makes sense for Silva is to fill in if there is an injury to either Rampage Jackson or Rashad Evans, which would probably also mean filling in on short notice.
This situation is interesting and Dana White could use it to his advantage by giving Silva the option of picking between Henderson and Marquardt for his next fight and if he wants his next fight to be at 205 then he has to face Machida. In the end, I would rather see Silva face Henderson and Marquardt Facing the winner, since the middleweight title has been on hold long enough while we waited for some things to shake out and for Silva to jump up to 205.