Set it on DS9.
Include the best of the cast from TNG, VOY, and DS9.
Lots of awesome stuff you could do with that. On a side note, is anyone else disappointed that there's zero Cardassian appearances in the TNG films? That they were series-long anagonists in DS9 but somehow not good enough for a film seems like a huge wasted opportunity.
Don't let the naysayers get you down. Your idea is very doable.
Do it like Peter Jackson's The Fellowship of Rings trilogy. One 6 1/2 hr. mega-epic story shot all at once then broken into 3 parts and released one per year. Financing could from an efficient Kickstart campaign.
To be clear, since we are talking the "best," Morn (with his deep soulful and expressive gaze) from DS9 would be included, right? Maybe as a scruffy rogue captain of a pirate ship!