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.