Layering even more MR and tenacity on to the boot slot is not healthy. Merc Treads already outshine most boots, and having a better version of it would make it very difficult to justify buying any of the other boots.

Boots of Maneuverability should remain 5 out of combat, but 3 in combat.

Perhaps make the recipe Boots of Swiftness + Boots of Mobility = Boots of Haste, 3/5 speed (since I think the gold/5 element is perhaps too strong a "late game" item). Or even make it so Boots of Swiftness + 650 = Boots of Mobility, then just remake Boots of Mobility (since the cost is a bit high requiring both boots).

Dislike the Mercury Treads and Ninja Tabi upgrades. The defensive stats are too high, I feel, for boots. Perhaps add Mana to mercury treads and health to ninja tabi? Or health regen?

Ninja Tabi + Kindelgem; Armor + Health + CDR + Nearby enemies have their armour reduced by 10.
Mercury Treads + Gold: (Active): Snares the target champion for 1 second. Can only be activated within 3 seconds of being CC'd. 45 second cooldown. *shrugs* Mercury Treads is already really strong, I can't think of a good effect to add that won't just make it a must.

honestly remove the gp5 boots. by the time you got those boots, it would be mid to late game, where gp5 would be useless. true you would get a little gold from it, but it would be too late to make up for the extra cashu had to spend to get the boots. the rest of the boots, with minor minor minor adjustments, would be awesome