While I completely agree with you, that begs the question of what a 'pure' Templar game would be. I imagine it would focus more on the Templar Brotherhood, or at least your guy being a commander like Haythem. So expanding on the Brotherhood mechanic from ACB, ACR, and AC3. No big chases or leaping assassinations, you could command your allies to blockade a road/alley to stop a running target. Give us a big crossbow or rifle to shoot down climbing Assassins, maybe expand on that with the ability to target legs to completely neutralize their climbing abilities. Instead of the counter mechanic, the combat could focus on brute force: You don't counter, you dodge, then strike. None of the elegant dance AC1 through ACR were known for, just brutal force smashing them when they're vulnerable.
And as far as climbing goes, I'd imagine the big step would be no climbing. Ladders, stairs, and so on would still be fine. Jumping and grabbing onto ledges, fine. But a Templar shouldn't be on the roofs. Instead, it could play into some kind of area mechanic, like the Borgia towers/Fortresses: Bust in, secure the area, it's yours. If an Assassin gets on the roofs in one of your secured areas, you can yell for rooftop guards to fire on them while you yourself are on the ground. Knocking them off the roof, potentially, or at least dealing damage.
Templar's are always portrayed as being the ruling force in an area, having command/sway over the guards. So why not give us that ability in a Templar game to make up for our lack of climbing skills? And put a bigger focus on indoors/rooftop access via stairs to give us a way up to the rooftops even without climbing abilities.