Hope I'm not the only templar fan here but even if I am, here are a few suggestions I hope will inspire the devs (even if just a little).
Before we get into the suggestions themselves, I'd like to say that to me, one of the most important things is that the class-spec have a strong connection to its theme, to its spirit, when it comes to its gameplay.... and boy is there a lot of that to work with when it comes to templar gameplay. So.... okay, lets get to the ability/passive suggestions.
1.
Crusader charge.
Charge in a straight line dealing notable damage and knocking back all enemies hit.

Yes I am a fan of the idea that mounts should also (occasionally at least) be weapons and this is one of these cases.
The tempalers were famous for many things, but their charges, often pure sacrifices for the sake of victory (lets face it, thats what being a melee dps in large battle is at the end of the day) are the stuff of legend and I think it would be a pure pitty not to bring them to ashes of creation.
Please imagine breaking the castle gate and being greeted by a wall of shields, a hard to crack nut that can make or break a game by itself. The solution? A templar charge straight out of a crusader movie to break the lines and get to the backline where the casters and rangers and healers are.
Additionally, I think a passive should exist that buffs allies once the charge is initiated, with an additional buff/aoe heal upon the charging templar's death.
2.
Selfless defender.
Summon your horse to evacuate a wounded ally from the front-line. The ally will become invulnerable for 2 seconds and will receive a major speed buff (the animation will show him on the back of a crusader charger) being able to flee from battle when seriously wounded. Once the buff is over the ally will receive a debuff increasing damage taken by 40% of 5 seconds (to ensure the buff isn't used offensively, which goes against the purpose and spirit of the idea).

Yes I know this is a reference to the hospitaller knights and not the templars, but they're both closely related organizations.
Either way, one of the things which made the hospitaller knights famous was them selflessly unmounting on the battlefield so their mount could be used to evacuate the wounded, while the crusader knight would stand, alone if need be, against the wrath of the enemy.
I think this would be an interesting game mechanic and would reflect the spirit of the class-spec well.
3.
Lionheart (passive)
When alone against multiple enemies, gain X% extra stats for every single adversary outnumbering you.

The most famous crusader of all times (okay ye he wasn't a templar but still, a crusader, same spirit) by far is the Crusader King, Richard the Lionheart. One of the things he is most famous for is his one man army stand, where he stud alone against a muslim army, roaring at them, challening them, and intimidated them into backing off.
The game will have a lot of objective pvp and one practice commonly seen in such games is leaving a man or two behind to defend a point of interest. Of course there's the risk of him always being overwhelmed by more players. But what if we combined the ''crusader spirit'' of the templar class-spec with a fun gameplay mechanic and made is so that the templar could fill a role where he is capable to face off against multiple adversaries better than other classes/spec would? Personally I think that would be a cool, unique thing for the templar, fitting well his crusader theme.
4.
Silver chains.
Bind an enemy to you making him unable to leave your melee range. Maybe even use the chains to drag the enemy in the direction you want to? Maybe the one dragging the other player in the direction he wants to is the player with most strength as long as un-cc-ed?
(okay so cool image for this, sorry... I mean, I hope the ones so far were cool at least)
Okay so this is another Lionheart theme. Apparently, our famous crusader king also had quite a bit of humour: Accepting the surrender of a cruel tyrant he defeated in Cyrpus, he swore to not put him ''in iron'' (to not humiliate the defeated despot by putting him in shackles), so Richard (interpreting the terms of surrender literally) put the defeated tyrant in silver chains.
5.
Retribution. (passive)
Upon death apply an aoe DoT effect to enemies around you, dealing damage equal to X% of the damage they dealt to you before death + an Y% increase [for each enemey which contributed]

Most people here probably know about the legendary cruse cast by Jacques de Molay, the last grand master of the Tempaler Order, against the king and pope who betrayed him for the sake of their own greed.
Again I think this would fit the spirit of the class-spec well and I think it would have good synergy with the front-liner, charger nature and theme of the class-spec.
6. As far as rotation is concerned, I think it would be cool to see a system where the templar applies a debuff (''Inquisition''?) to the enemey and, after a certain time, the templar can use an offensive ability (''Judgement''?) that will affect the enemy in different ways depending on how the enemy behaved.
- If he was a ranged attacker which kept kiting from long range --> He will be judged as a coward and suffer a debuff which greatly increases damage dealt to him from all sources for some time.
- If he simply dealt a lot of damage directly --> He will be judged for his viciousness and suffer part of that damage as retribution (up to a certain limit)
- If he is an enemey healer --> He will be judged as an accomplice and will have a dot applied to him dealing X% of the damage he healed the templars enemies for (up to a certain limit)
- If he buffed enemies --> He will suffer debuffs amounting to the complete opposite of the buffs he gave out and his ability to buff will also be tained for a while, throwing out debuffs instead of buffs for that period.
- If he debuffed the templar's allies --> He will have all those debuffs stacked on himself as retribution (up to a certain limit of damage and debuff time)
Meanwhile another ability (''Crusader strike?'') should be used to increase the duration of the Inquisition.
The Inquisition would be a suble effect and would tax enemies which simply don't pay attention and let the templars get too close.
7. Holy Lance
After a short cast time charge into a straight line, stunning the enemey you hit, while dealing some damage

Okay I stole this from Darkest Dungeon, its pretty cool and I'm sure they dont mind

I mean... the templar is by definition a knight and knights are supposed to not be the most mobile (they were in fact very mobile despite baseless current misconceptions) or nimble but if a gap closer is really needed, maybe this would be a good idea.
Just some random ideas.
Very eager to hear yours templar fans.
Who knows, maybe a dev who is a templar fan gives these a read and we end up contributing to the game and our future experience.