Here's a proposal for how to do this ultimate mixed-cast movie we're talking about:
Go ahead and write an epic story that involves all of the living actors and their characters, or as many as can coherently be included. Hell, include the nuTrek crew IF it can make sense to. Use canned footage and CGI for the important parties like Nimoy and Doohan and Yelchin - but make sure it's done well. Don't worry about the length of the script, because if this turns into a trilogy, everyone is going to be fine with that, I promise.
Film anything and everything that can be filmed for that, now or ASAP. Film some optional footage for the Discovery crew, too.
Once Discovery has ended, hopefully as a success 7 years or so down the line, then come back to this material and add to it - new footage with all of the actors older.
The movie/trilogy is a time-hopping adventure that deals with some massive last threat occurring over the span of all of Trek and even maybe the Mirror, Prime, and Nu universes. And, some characters die in meaningful sacrifices, but in the end, the effects are that 1. the Temporal Cold War is cancelled out and time travel itself is made unreliable and uncontrollable (some sort of weapon from the Continuum?), and 2. we're left with the idea that true and lasting peace has been brought to the Milky Way - the Dominion, the Voth, etc, everyone we know about - at least for a while, probably due to cooperative efforts to rebuild from massive amounts of destruction or tech disruption because of the weapon. And our heroes are left with naught to do but to either retire in peace or to actually *explore* the unknown systems of the galaxy - "happily ever after".
And then the next series is either that - a new series for the actors that want to sign on - or an entirely new crew with almost no baggage, exploring a neighboring galaxy in the late 25th/early 26th century
One more thing: kidnap the marketing geniuses for the Marvel movies and get them to work on this.
