1) J P Thirion (1998), Image matching as a diffusion process: an analogy with Maxwell's demons. Medical Image Analysis 2 (3) p. 243-260
Effectively optical flow - looks very easy to implement and doesn't require a great deal of understanding about optimization.
2) Kybic, J., & Unser, M. (2003). Fast parametric elastic image registration. IEEE Transactions on Image Processing, 12(11), 1427-1442. Institute of Electrical and Electronics Engineers
More closely ties in with the work in the codebase so far - partially implemented already.
Two interesting approaches for this:
1) J P Thirion (1998), Image matching as a diffusion process: an analogy with Maxwell's demons. Medical Image Analysis 2 (3) p. 243-260
2) Kybic, J., & Unser, M. (2003). Fast parametric elastic image registration. IEEE Transactions on Image Processing, 12(11), 1427-1442. Institute of Electrical and Electronics Engineers