Things like rivers freezing just aren't going to happen. It requires new code, and then you have to start considering, which rivers etc though I suppose that since travel times all increase we generally assume that all continents are located such that they have European style winters.
Here's a simpler proposal on travel times which would not require new coding: instead of making travel longer across the boards, make it so that travel times are longer in mountain and hills in winter only, longer in plains in autumn only (due to the mud), and longer on sea in the spring (let's say... icebergs. yeah, icebergs. Or sea monsters).
That way you have a cycle, but you have no season where every single way is blocked.
