If Australia selected an air independent propulsion (AIP) technology for conventional diesel-electric submarines it would be on a new-construction Future Submarine rather than being retrofitted to the current Collins subs.
It is unknown whether the two Type 218SG subs being built in Germany will utilise fuel cell PEM or Stirling engine AIP. If it is Stirling (an AIP Singapore is already familiar with in the two Archer class) then that might be one reason that the new submarines have been called "218" rather than "214".
Methanol defragmentor is the latest AIP being considered - German, France and Spain may be ahead in this.
Indian DRDO Fuel Cell - A March 2014 report indicated DCNS might install a DRDO developed AIP based on hydrogen fuel cell technology on one [or more] of the six Scorpenes beinge built at Mazagaon Dock under Indian Navy Project 75. This was in preference to an older version of French MESMA being offered for the Project 75 Scorpenes. The DRDO fuel cell might be also be built into the second line of Indian future conventional submarines under a different project, known as P75I.
Now Saab and FMV's Stirling AIP system.
See a French DCNS announcement of a "second generation AIP" (more fuel cell type - not really a MESMA development) at DCNS' new submerged SSK solution - Lithium-ion battery and 2 AIPs of November 6, 2014.
Russia hopes that it could develop the 4th generation Lada-Amur class have been discontinued because Russia has been unable to develop satisfactory air independent propulsion (AIP). This makes prospects for an Amur class from dim to nil.
Nuclear reactor ("AIP") on submarine (Source - Excellent explanation of nuclear technology)
Using any of the AIP technologies would involve lengthy selection processes. Extensive foreign and local training would be required for the technicians, engineers, officers, crew and officials required to maintain, use, administer and repair these technologies. All this takes years and in many cases decades. AIP technologies need to interact with the complete submarine weapons system - so can't only be seen or assessed in isolation.
The Submarine Research Center reference on AIP and U31 at http://adjunct.diodon349.com/Uboats/answers_to_questions_about_the_u_31.htm