Alright. Just thought I'd drop in and give my opinion.
First off: I can't see why you'd want to make a Lego game, considering that Legos are like....... toys....... and.... I don't know. It just seems strange to me. I mean, hell. I love Legos as much as the next guy. I've got boxes and boxes of the suckers, but a game just seems ridiculous, unless it's based around building.
My ideas:
1.) A lego simulator. Basically, you create a lego world. RMXP/VX difficulty? Quite near impossible, I'm sure. But still.
2.) A kind of.... do what you want with your Lego life game. Where you'd basically make a character, move out of your parent's and start meeting friends, building stuff for your new home... or just building your new home... maybe you'd get a job... Hell, basically Lego Life. I think it'd be neat.
3.) Anything that isn't already a movie. Yes... I play all of the Lego movie games with my cousins, but they're pretty young, and since the actual movies and movie games (not legos) are kind of violent and graphic, their parents think that the lego versions are great choices. I have to agree. But depending on what audience you're aiming for, I'd really suggest staying away from a fan game of a fan game of a movie.... if that makes sense.
4.) Something new. Maybe a futuristic, very creative and new extreme sports game. Where you find/buy/win/earn parts for your hoverboard or whatever. You can build on, customize your tools, whatever, ya know? Something different that still incorporates Lego Building.
5.) I particularly like the cyberpunk genre, even though... I don't really know what cyberpunk is, or where it came from, or how it really works... I just know that the games, concept art, and the basic ideas I've seen here on the forums are awesome. Something like that would be fun.
Good luck. There are my ideas. XD Have fun with it.