Hunh...okay.
Well, it's a tough idea to balance out customization with combat elements. If I were to describe it best, I'd say it'd be a mix between Soul Calibur's Create-a-Soul, a kind of fashion-oriented generator (with stuff you can import yourself like tatoos, makeup, jewelry, clothing, or dress designs or hair style and body types), elements from "Artificial Girl" series (though I hate their designs as it looks TOO artificial, lol), and something akin to a psycho-religious alignment system (that is to say, a person's sign and birthday or their personality affect their overall abilities including things like likes and dislikes based on genre of taste).
A more elaborate way of saying it would be, the emphasis is on designing your ideal girlfriend, from clothing to personality type which ultimately affects her growth and abilities. For example, she could be introverted/extroverted, active/passive, optimist/pessimist, idealist/realist, and you could adjust things like how sensual or reserved she is, formal or perverted, open or secretive, domineering or submissive, and depending on your settings that affects her growth and talents in certain areas, while things like her sign, likes and dislikes affect how well your interaction with her will be on certain events.
And how well she gets along with you will determine her strength and ability growth (somewhat like Nintendogs has you looking after a pet) in various areas, as well as things like hidden talents (such as singing or the like). Also making a girl who has a wide variety of personality range would create mood swings.
Certain clothing would boost her personality attributes and parameters like equipment would, as well as her relationship with you.
Lastly, there'd be four key events that further boost the relationship or are detrimental to it:
--Dating Events: Could be stuff like watching a movie, giving her a gift, celebrating a holiday, or going someplace like a restaurant or concert.
--Battle Events: Where she actually has to use her powers in conjunction with your control of her abilities. It'd be something akin to a 3D battle system most likely, like Power Stone or Smash Bros, with elements of KOF/GG/BB perhaps. This is where you could possibly play your friends and compare girlfriends. There'd also be action game and story elements in these type of events to further the main story. Think GG-Boost-like. Oh yeah, and things like equipping certain weapons would affect their fighting style.
--Challenge Events: These can appear anywhere in the above two events usually, or anytime you want them to. Things like drinking/eating contests, beauty contests, task challenges. Basically a large group of unique minigames, possibly something as spread out as Wario Ware.
--Passion Events: These would be locked at first, but as you progress, you'd be able to unlock them, and by doing them sometimes unlock a special ability or gain a large stat boost. Somewhat like Challenge events, but not entirely based on your performance in the challenge, hehe.
Anyway, that's the ideas I'm working with...I'd go into further detail about the level of customization I'm thinking about, but it's a huge list.
One more thing: about the premise for this game...it'd be something like a magical girl (but with somewhat darker elements) who needed the power of her boyfriend to fend off evil or something...yeah I'm corny like that.