Marth was important in two games. He drove the plot alongside Nyna.
Ike was important in both of his games. He drove the plot alongside Elincia and Micaiah.
Chrom drives the plot for five maps (Prologue - Ch.4). Then Emmeryn steals it (Ch.6-9), then Gangrel (Ch.10-11), then Walhart (Ch.12-20), then Avatar and Lucina steal it until the end of the game (21-endgame).
I'm not sure what connection you're seeing.
I'm sorry but I can't buy that logic, it's like saying Link was totally unimportant to Twilight Princess because Midna had such a strong presence, and really did steal a lot of the show. Chrom is directly involved in the events of pretty much every chapter, even the child recruitment ones are opened and closed with comments from Chrom in most cases. More than one character can have a strong presence in scenes. Same can't be said for any of those characters, even Robin. I can see the viewpoint that he might not be the most exciting or interesting character but you can't just sweep the fact that he is the main protagonist under the rug, he drives the plot and without him they would have failed, just like they did in the future. And how does Lucina steal the show till the end of the game? After she is revealed her importance is greatly diminished she makes a few comments here and there and really the only big thing she does is her "judgement" scene which pretty much is there for added drama and doesn't actually have an affect whatsoever. Other than that she just kind of runs around going "oh gods it's grima."
As far as move set potential for either Chrom or Lucina rather than focusing soley on weapons they can use a better place to look is the skill system. From what I understand (haven't played all the Fire Emblem) Awakening was the fist game is the series where you really had the capability to make a deeper custom skill set by re-classing and such. Seems like in addition to the whole dual guard and pairing up concept this was one of the other different features that Awakening introduced. I think there are a couple cool ways you could implement this. Pretty easy to see how that would work but would still set him apart from the other lords a bit. A more interesting concept to me would be something similar to the idea that Kikaru posted a few pages back but more expanded. Chrom's down B could be "equip skill" and depending on which skill you have equipped a number of things can change. These are just basic ideas I've though up on a whim so I'm sure other people could come up with better ideas for this concept here are a few examples (excuse my butchering of the proper move connotations, I've not familiarized myself with the proper way to write them)
Aether: The default equipped skill, all this would do would make Chrom's Up Special work similar to how it did in game. On successful hit it would heal a small amount of % and essentially put a debuff on whoever it hit "reducing their defense" which in smash terms would mean they take more % from attacks (from all sources, not just Chrom) it wouldn't be an overly long debuff but even a 5-10 sec debuff like that could be useful.
Counter: When counter is equipped all physical attacks done to Chrom do a % back to the attacker. The tradeoff for this could either be something like ranged attacks now do more damage or that Chrom loses a theoretical ranged attack. For this example we'll say his side special is a Lance Throw, if you have counter equipped you lost the lance throw and your side special is replaced with just a lance thrust.
Pavise: Take half % from attacks but makes Chrom move slower (without gaining any power). Alternatively this could replace one of his other moves when equipped and instead be an active skill similar to Pit's Mirror shield except without the reflect property.
Limit Breaker: Chrom breaks his limits, pretty much all his attributes are increased slightly but he damages himself on attacks.
Galeforce: All Chrom's attacks do double % and Chrom moves faster however moves now no longer do any knock-back and Chrom is lighter.
Really basic ideas for now but something along those lines focusing more on the Skill system than what weapon he used would both make him more unique and represent a pretty big part of Awakening. It might come off as kind of gimmicky but apparently a character has to be in order to be unique in some people's opinion. Obviously an idea like this could work just as well for Lucina too.