I think Nemesis destroyed the movie franchise because the producers sat on their laurels. They expected it to follow the same odds, even numbered movie trend. They bragged about it, and expected that the info that it'll be action pack will give it a bump in the box office.
Riker should have commanded his own ship, and the Titan's engines should be disabled during the final battle (avoiding the need for the collisions course).
The whole plot made no sense, and took an excessive amount of time to introduce half heartily, so likely better to replace Shinzon's with someone from TNG's past like Sela. She should be the one imprisoned on the moon, and wants revenge on Picard and Data for spoiling her invasion plans of Vulcan. That would at least replicate elements of TWOK.
The B4 plot was a tangent that didn't really need to be there. It allowed for a backdoor plot to bring Data back from the dead. Realistically it would have made more sense if they just created a beat where Gordi talked to Data, where Data explained the progress he's had a replicating his own positronic matrix since Lal. It's almost complete, and he develop a nano-tech process to simulate natural aging, however the matrix is not a perfect replica and the positronic matrix will degrade over time. If he just transfered his data over to the new body, to see what it would feel like to grow old and discovered the problem. The clone is now off as he works on the problem. The clone will eventually die at an undetermined time as degradation appears to be random. If not fixed the android will die from an equivalent of dementia. (Solves the Brent Spiner aging issue, and gives his character a much needed terminal disease) Both Datas goes to save Picard, but in a transporter malfunction (from the Titan, as the Ent's transporters were off line), only the clone Data and Picard survives the transporter accident.