On the Mobility Proposal
It is my sincere belief that the proposed system would cause much chaos to the balance of the game. However I like where you are coming from. I have formulated an idea that I think could bring our ideas on mobility together
As follows:
add -1 movement cost to Flanking3, a cavalry only promotion
add -1 movement cost to Sentry2,a Recon/Archery only promotion
boost Sentry-line by making Sentry1 require one of either Combat 1 or Mobility 1. Make it a choice.
A way to give an additional barrier to Sentry 2 would be to make it require not only Sentry 1, but also either Mobility 2 or Combat 2. You could go Mob1, S1, Mob2, S2, or C1, S1, C2, S2.
In this way you get your 'Mobility 3' for your Cavalry and Recon (and occasional archer). This inadvertent buff to Archery would allow for further choice between the Longbow and the Champion ... a decision that has long been ignored due to the perceived superiority of the Champion unit.
Mobility 1 is still at Horseback riding ... and we will nerf Mobility 2 by making it only available after Stirrups is researched.
Mob1 will no longer be made available to Seige weapons.
Mob2 will be available for Recon and Cavalry units only.
In comes 'Logistics' promotion. A promotion previously awarded to Great General led units in BTS. Could have a different art. This promotion will not work like its BTS version of +30% withdrawal, but INSTEAD it will be giving +1 movement.
The beauty of Logistics is that it will only be made available to Units that cannot buy Mobility 2. It will also be made available to units that cannot buy Mobility 1! So Logistics will be your 'Mob2' for melee, archery, arcane, disciple, etc ... and it will be your 'Mob1' for your Siege and Golem units.
The trick of Logistics is that it will not be made available until the ENGINEERING tech. Still making Engineering a popular tech for those that wish to move more traditional armies (but largely useless for practitioners of the Lightning War). Certainly Engineering is a step on the way to Machinery ... but users of Cavalry and Recon won't see much need for it, unless they are a pure Recon user and do not wish to tech Stirrups for the promotion alone.
I believe that this combined method will make for more interesting strategies and will continue to uphold the balance of the game.