I think that 7DMC will be better than Mermaid (at least better than the Little Mermaid ride in DCA which has a half-finished second half and a lot of plastic fish that don't move).
I really like the landscaping they did on the Beauty and the Beast area of FLE, if 7DMC is the same, then expect a very nice area to walk around, such as the Norway Pavilion during Flower and Garden. The ride mechanism is new, and will be interesting, if you get the sensation of careening around the outside of the mine, then that part could be cool.
The negatives are that TDO didn't hit the ball out of the park, turning FLE into a "must see" for everybody. I think that if they did a massive 7DMC with more show scenes, and which weaved around the FLE, instead of just plopped down in the middle, it would have been a hit for the ages. We've got the dwarfs in the mine, and a cameo at the end. Really frustrating because you kind of get the sense that WDI didn't go all the way ( or likely the budget was whittled down), and yet another $50 million and we'd be looking at an E-Ticket.
Maybe Disney doesn't want such a big E-Ticket in fantasyland because the line could get really long and muck up the traffic flow, though they could easily stretch the line around the attraction.
Normally, when Disney does a "mountain ride", they are pretty grand affairs which have broad appeal and stay popular throughout the decades. Everest is still popular despite a broken Yeti, as is Splash and Space. I don't think they needed to make 7DMC necessarily have a massive profile, but more show scenes would have been OK, and would have pleased guests of all ages, even if the witch sort of got the axe.