Are there any ways in 3.5 to play as an an intelligent weapon that possesses a host?
I suppose. I don't have my DM manual with me, but IIRC the rules for creating intelligent objects allows for different power levels, that include starting PC level.
A cool DM will allow it, and an intelligent DM will find ways to make it balanced.
The biggest question IMO is : How do you justify the sword gaining levels ?
On a sidenote, there is a quest about an intelligent sword being run on /tg/ right now >>44610661
I the sword just has whatever levels and classes the intelligence has. With a spell like ability to temporarily learn knowledge of the host, but no permanent knowledge of the host.
If there's an established structure that isn't horsefucked I'd like to use that because it has a better chance of approval by the DM.
No that I don't think I couldn't make a case for it, it would just be easier to tweak existing rules if it doesn't work the way I want than homebrewing an entire system.
i'd go with >>44612052 on this one if i were you, OP
Just checked the 3.5 dungeon master guide, play an intelligent/cursed item with a good ego score, and you'll be able to control your host most of the time.
The book got powers, skills, fight for control, and drawbacks. It's everything that you need.
How about a sword of average intelligence but somewhat robotic personality, who doesn't fully control its host or impart any fighting skill onto them, and tends to forget that humans feel pain and need sleep?
Why don't you just say "by the way, my character is controlled by his intelligent sword"?
There's no way your DM will be able to disprove you. Even if he says "Nuh uh" you just say "well, my character thinks he is".
You don't need ANY rules for this at all.
The details, as best as I recall, are that the Ninja Spy prestige class grants Alter Self at will, which a Warforged can then use to transform into any construct within one size category of itself. This includes, as mentioned, the various animated objects.
Because the sword would be magical and that would be a potentially significant advantage at lower levels giving him drawbacks like limited control when not being worn or held by his host would be interesting.