Engineering is always an evolving discipline. By that I mean the correct time to raise an issue or flaw with a plan, in engineering, is always "right now", because the sooner the problem is raised, the sooner it can be fixed.
Aside from the obvious reason that if your suggestion is actually better then you'll waste time thinking about a subpar solution, the other issue is that perhaps it is you who is missing context on this application you're being asked to build. If you walk into a meeting and you spend a bunch of time outlining another possible solution that solves X problem much better than the proposed solution, but then someone comes out and asks "how about Y?", when you didn't even know Y was a thing you needed to think about, then you're going to look really silly and waste everyone's time.
So here's what you should do: Whoever is in charge of spearheading this new feature, send them (and only them) an email asking for more details about this new assignment, and mention you have another idea and ask for feedback on that idea. Don't assertively say your idea is better or whatever, because you may not have all the facts. Simply send your idea and ask what they think of it. They may come back and say "oh, we also need to solve Y", and maybe your solution doesn't solve Y, in which case you've just saved yourself a whole bunch of time, hassle, and embarrassment versus bringing it up in the meeting.