Would it be a good idea to place a couple of dense mountain stages in the first week?
I'm talking about a Bonette/Isola or Galibier/Granon stage in week 1?
It could sort the GC order early and force the riders to attack on lesser stages in week 2/3??
Or what is the ideal 'pacing' for a Tour?
You can try it for sure but, because these stages are the ones heavily marked by the GC favourites, it's likely the Tour gets decided on that one day, and its just a fight for minor places after that.
I think the 2020 Giro route did a really good job of pacing. You have some early summit finishes and mountain stages that set a GC hierarchy but aren't complicated enough to completely decide the race from the get go. Then some transition days and the first really big summit finish only by the end of the second week to keep the tension until late in the race even in the worst case scenario. The big mountain days come in the 3rd week but it's not completely overloaded (really only 2 big days to Cancano and Sestriere separated by a flat stage). There is always a risk the race is decided before then but you maximize the potential for exciting racing throughout the 3 weeks.
You can replicate this style for the 2026 Tour. First couple of stages are perfect for setting a hierarchy without deciding the race already. If you keep the Pyrenees simple and don't overdo it by using lesser climbs like the rumoured Guzet-Neige, its unlikely Pogacar is going to hard drop everyone and gain minutes deciding the Tour already in the first week. Massif central and Vosges are perfectly placed for several transition days (a couple GC relevant like Le Lioran to keep us on our toes without doing too much), and then the Alps are where its all decided with a couple big days to Solaison and the Alpe. Very simple frame below:
1-3: Depart
4-5: A couple mountain stages on the easier side, potentially allowing a solid breakaway to go and take the yellow jersey for a few days for example.
6-9: transition stages
10-13: Le Lioran, perhaps a TT if not earlier and more transition stages. Its even possible to return to the white paths around Troyes if you really want or to do something like Le Creusot 2021.
14-15: Vosges to bring the GC battle towards a climax without being decisive yet.
16-20: Alps to decide the GC, with some sprint / transition stages somewhere in the mix (probably towards the start of the week).