You're definitely right that children would love that sort of thing.
My perspective, as a parent of a 2 and 4 year-old, is that it would just be another stuffed animal. Both of my kiddos have their own massive pile of stuffed toys that people have given to them over the years. Much of them are forgotten about, and new stuffies are replaced after a short amount of time when a new one comes along or when they get bored of it. I can't tell you how many character stuffies my kids have been excited about only to all but forget about two days later.
My kids would love it, sure, but for them it's just the security of a soft cozy friend that seems to be the most important thing. What it looks like/who it is seems to be secondary and entirely mood dependent. There's never been a moment where there's felt like a void because Hobbes isn't in bed with us, and he wouldn't be accomplishing anything that the rest of their stuffies aren't doing already, save for padding Watterson's retirement - which I think is part of his point.
But everyone's different, so YMMV.
Edit: I suppose if one of my kiddos asked for one and it was super important to them, it would be kind of fun for us to make one from scratch!
Probably unapproved third-party merch, like those bumper stickers where Calvin is pissing on something. Watterson is famous for having always been anti-merchandising.
You can merchandise things without going overboard and losing your soul.