so a human fighter doesn't cut it. A fighter that used to be human but is trapped eternally in a suit of armor in the body of his clans most hated enemy? thats the ticket.
As far as your 'trapped in a suit of armor' thing, why not just make a Fighter, slap him into a suit of plate mail, and say that he's 'cursed' so that he can never take that armor off? Doesn't require a template but you get to keep the backstory. Really the only downside would be the penalty you take for sleeping in armor, but that doesn't really sound like an issue for your DM.
well theres a homebrew feat called "black Knight" on a wiki for DnD that were allowed to use, long story short, you get a masterwork full plate or free, but can never remove it, but you take no negatives for sleeping/climbing/swimming in it and enchanting it costs half the price (not something we really care about since in our group magic is NOT something we enjoy on our fighter/knights). As for the advice yeah it would work but...it's a little uninspired, how is it dramatic or engaging? normally combat isn't he bread and butter of our games but the role playing and characterization.
And my first question would be: what's the difference between your initial post and my initial response, other than I didn't use templates? The character is trapped in a suit of armor, which sounds like exactly what you going for. To me, anyway, that can and should lead to a lot of roleplaying possibilities without needing to add some other abilities on top of it, which you would most likely get from a template.
If nothing else, because the character is cursed to stay in the armor forever, side plots could be attempting various means to break the curse, which you could set up with the DM to not work because of something or other.
Secondly, how is that
not dramatic or engaging? Your party is invited to the King's court, but
all weapons and armor must be removed. Well, that could be a problem. Unless you roleplay it with the King or the King's Captain of the Guard. Image the look on that guy's face when you raise your visor and there's nothing inside! Except there is!
CotG: SORCERY! Prepare the gallows!
Party: NO! No no no! A curse! He/I has/have been cursed by a Trogdorian (your hated enemy) witch to forever remain in this suit of armor!
CotG: Hm. Well. That changes things. WITCHCRAFT! Prepare the gallows!
Party: *collective headslap* No! HE'S not the witch! He was cursed BY the witch!
So on and so forth. The King's Minister or Pope or Head Cleric or what have you is brought in and finally, YES, it is determined that you are indeed cursed and are NOT a Sorcerer or a Wtich. But THEN any Sorcerers or Witches (arcane spellcasters in general) in your party are offended, because APPARENTLY THESE PEOPLE ARE IDIOTS and think that Magic = Bad! Well, there's ANOTHER set of roleplaying possibilities.
If I were to go this route, I would honestly take the penalties from essentially 'being' a suit of armor because that, too, can lead to good roleplaying. Rolling badly during an encounter? Blame your character's performance on not sleeping well because he's not really 'human' anymore. Rolling well? 'Gee, not having a body to worry about makes things a lot easier!'
Or, if the 'character' is just the animating force of the armor (but you still use the character's stats for everything), think about all the things someone used to having a body would miss! Food! Drink! Sleep! Sex! Sensory deprivation can add a whole other level to your roleplaying. "I think out of all of the things I miss from having a body, I miss pork chops the most."
Just because there are no templates involved doesn't mean roleplaying is out the window. In my PF game, I'm running an Elven Monk who was captured by Drow and learned their 'Spider Style' of martial arts. As everyone in my group (except me) has studied various forms of kung fu, it was fun for us to take a few minutes to figure out how that style would work (lots of pressure point attacks and grappling). That right there gives me something to roleplay with. If someone comes up and is talking to us and wonders why the Elf is staring at him, it's because I'm studying that person's pressure points based on what they're wearing.
He also has a hatred of Drow, which was interesting roleplaying when my DM changed one of the NPCs in the adventure path we're running into a Drow specifically for that reason. I ended up not killing her. Just roughed her up a bit when she got uppity.
The opportunity to roleplay is there, even if the templates aren't, is all I'm really trying to say.