The BBC could make a fortune out of allowing other channels onto iPlayer. UKTV is one example but something like Channel 5, who probably struggle to get people to stream their stuff normally, would get a huge amount of extra exposure.
Yeah. If there were to be one unified platform for the BBC, ITV, C4, C5 and UKTV then iPlayer would be a very strong competitor to Netflix. In an ideal world they could monetise and add adverts to some programmes and have the BBC's remain ad-free.
There already is - BritBox. They use their own free services for recent shows (usually 30 day catch up, though at least iPlayer has boxsets too), and BritBox for archive shows. For example, BritBox has 1963-1996 Doctor Who and iPlayer has 2005-2020 episodes. ITV Hub has Coronation Street episodes recently aired on ITV or ITV3, and BritBox has different older episodes. Since a lot of UKTV shows are also BBC archive, they can also turn up on both iPlayer and BritBox.