(a) some sort of Martian civilisation (the Martian in BP has some sort of exoskeleton though it is not clear IIRC whether it was made with Martian or Earth technology)
(b) Venus as some sort of swamp planet with
(i) breathable atmosphere by humans
(ii) edible food by humans
(iii) some sort of intelligent civilisation
with this not obviously contradicted by science?
Although... the first thing I remember thinking about such stories is that surely one has to postulate either a wild coincidence that the civilisations would have to grow up around the same time or the Martian and Venusians ones would have to be in a steady state for hundreds of thousands / millions of years.
2. Has anyone come across any alternative history novels (ideally at least semi-worth reading in which)
(a) the Morgenthau Plan was implemented
(b) the Ottoman Empire joined WWI on the Allied Powers side
(c) set in the modern day in which Germany won WW2 but suffered a Soviet Union / Eastern Block style collapse in the late 80s / early 90s or thereabouts
(not all at the same time).