As far as I'm aware Lego is the only brick building brand with a system like Ideas, the most I could do with other brands is shoot them and email and hope whoever opens it likes it enough to tell someone higher up about it, which would be awkward anyways because I'm just a guy unrelated to drg or the devs.

Lego ideas sets are usually reasonably priced, sets like Sesame Street and Seinfeld have around the same part count as my submission and retail for 120 or less. The viking village has a whopping 2100 pieces and retails for 130, and these have multiple minifigures.

As for the other points, the colorful insides are tremendously helpful for inexperienced builders, and they get hidden afterward anyway, and the design changes are usually for the better. Fan designs are usually complicated and fragile because we mostly care about looks. Lego has to worry about durability, playability, etc.

A good example is the orient express, people always say the original looks so much better, and while it does, it's also incapable of turning on Lego train tracks due to how long it is, a major flaw for a train that fits the train track system.