Hard to say. From a personal perspective, if we are getting Lesnar/AJ at SS then I probably don't need to see it again at Mania. If it is an unfinished story or so amazing that it needs a sequel then building it through a Rumble win could be the right way to go.
From a financial standpoint, giving Cena a Rumble win and match with Lesnar makes the most sense. It will also give us the best Raw/SD build as Heyman and WWE can build Cena as truly an underdog since Cena has a recent history of losses and the squash hanging over his head. Plus who else is going to draw with Lesnar. Reigns isn't there. Braun might be able to get there. No one else feels worthy, while Cena is becoming more and more of a mainstream star. There is also the Flair record that can give certain people the feels if done the right way. How cool would it be of Cena tried to start off the match with a spear in a similar way to how Goldberg managed to defeat Lesnar? What if Cena found a way to block Germans? These are match spots that could play well with diehards while the casuals eat up having the UFC guy battle the star of Blockers.
Otherwise, a healthy DB or The Rock makes sense. An HBK return, Stone Cold, not Goldberg again. Roman needs to stick with The Shield. At what point between the illness and suspension can Roman get the injury (i.e. absence) prone label? He has missed a lot of time suddenly over the past few years.