If you have the Pyrenees last, the transport connections make it much tougher to run the mountains all the way to stage 20 than when the Alps are done last. The main option therefore is to have the mountain stages at the start of the final week then transition up towards Bordeaux with a less dangerous stage or two before the final TT, be it around that area like in 2010 and 2014 or after the transfer closer to Paris like 2012.
The best we can hope for to prevent the cursed ASO "penultimate weekend flat strage extraordinaire" route design is that they arrive close enough to the Pyrenees that they can do stage 13 to arrive close enough that stage 14 (which will be the penultimate Saturday) can be the Balès-Superbagnères stage and so another mountain stage can come on the Sunday. That way they can have the TT after the second rest day on either 16 or 17 depending on if they want a third Pyrenean stage, then they can transition northwards to allow us a medium mountain stage to the west of the Massif Central on the final Saturday, where something like Super-Besse would come in handy (especially if tackled from the side seen in 2008, like
this), or something like the Super-Lioran stage this year as a finish; or they could go toward Clermont-Ferrand or so forth or so on. Remember, this is the final roll of the dice with only Paris to come.
14 - Superbagnères
15 - other Pyrenean monster stage (as described possibly?)
- rest day -
16 - easier Pyrenees stage (probably starting in Pau? Would suit an easier finish climb. Marie-Blanque - Aubisque - Cauterets or even Couraduque after this year's introduction in the Route du Sud?)
17 - transitional stage northward towards the Gironde
18 - CLM somewhere around Bordeaux or Saint-Émilion
19 - hilly stage towards somewhere like Brive-la-Gaillarde or Tulle (respect for Abdou!)
20 - intermediate stage to somewhere like Super-Besse or, transport-wise, perhaps better to be Clermont-Ferrand with Ceyssat-Moreno as last climbs or finish at Circuit de Charade
21 - Paris parade