I like the format of the Hero and Monster Collections for Descent: https://www.fantasyflightgames.com/en/news/2014/11/7/hero-and-monster-collections/

Wouldn't those be great for Imperial Assault?

Depends. I kinda like how you can buy 'just' what you want (though access to Command Cards can be an issue). The HMC for Descent is largely based on 1st edition content. They would probably make a lot of sense for Campaign players (as they do in Descent) but I'm not sure how they would improve Skirmish play / availability.

E.g.: Fast access to all Heroes from Rebels or Rouge One without making a campaign.

I would think any established character from Rebels or Rogue One would not be a playable hero in the campaign, just an ally.

Any playable heroes would need their hero missions, so building a (mini)campaign around those would not be that much more work, just flavor text which does not need any additional internal and external playtesting. So, I don't think a hero pack would save FFG much compared to a small-box expansion other than not needing to include mercenary and imperial units (which would perhaps be a wasted opportunity).

I mean, this doesn't seem wildly different than stuff like Wave 7, other than that it's all in one box. I mean, if it's cheaper than I'm all for it. But I do see this as an issue for non-completionists.

For instance, if there was a Wave 7 bundle for $25, that's a little cheaper than buying all three of the packs offered. But if an Imperial Skirmish player wanted only to have the GI, he's not saving any money by buying the pack.

Also, I'm led to believe that these packs only contain the figures. What I like about the current packs is that they have several goodies- command cards, a mission or agenda set, items, a skirmish map. There's a lot of good stuff in each one.

The only reason these HM boxes exist was to monetize all of the old content from Descent 1st ed to 2nd ed players that missed out and are super completionists. Any Descent player will tell you these are probably the last items you should consider purchasing if you are looking to maximize your ROI.

As for releasing existing IA figs in these types of bundles stripped of all their campaign content, that would be a really bad idea for FFG, as they would basically be doubling their production values to compete against their own product. This would only make a very tiny portion of the customer base happy. Skirmish only players that don't already buy the box expansions but would if they were $10 cheaper and stripped of 50% of their content (including tiles) are not a large enough demographic to be worth catering to, assuring that type of player even exists at all.