Like most sequels created due to audience demand, much of the same equation is just spread out over thinner bread. Okay that metaphor mixed. Sequels are oftentimes the same butter spread out thinner over a new slice of bread. Yes. You know what I mean.
What are the sequels that improved? I can't think of any off hand. Movies like terminator 2 and aliens we by different directors I believe. Books: pinball, 1973? I wonder if some theorist can explain why there are basically no literary sequels that exceed the original but film examples exist.
in ny case, RAMA II has a somewhat darker edge than Rendezvous (is this the influence of Gentry Lee?), without its stylistic/political unity, a world in which 20th century politics seem to be reviving versus Rendezvous' most striking "Hermans" (mercury-dwellers, who are the hot-headed and unpredictable wildcard of the interstellar United Planets). suddenly we're back to france and tories in England again, and although it allows for a sympathetic heroine, the overall mood effect is a step back.
Still, in strict publishing terms, "the mystery deepens" as they say. who are the Ramans? what do they want? you get some answers, and then you get more conundrums.