I think someone should make a landline replacement mobile phone. Something that's meant to stay in a charging cradle at home, but can come out and about if necessary. Now, a lot of people would find that an incredibly silly and useless thing to have, but I think for retirees and people who spend a lot of their time at home and are still making the transition to mobile phones, it would work really well. Instead of having two phones, or just the landline, they would have one phone that looks and acts just like their old home phone, with the added benefit of being able to take it with them when they go out.
The funny thing here would be the marketing. As far as the phone company would be concerned, this is a mobile phone that usually stays put, and they would treat it as such. But the main benefit to customers is not the usual mobile phone sales pitch, but the familiarity of the phone while it's at home. To them, they're not buying a mobile phone at all. They're buying a cordless home phone with unlimited range.
Mokalus of Borg
PS - A generation from now, this idea won't make sense.
PPS - And it's a small niche to fill, so we probably won't bother.