When the arrow is to the left, it means from the previous age.
Should we conclude that inno will allow you to gain resources and units of the next age? If so, I'm afraid it will ruin the game.
Resources of next age won't do anything unreasonable. Presumably it's capped at the current top age (like previous era in bronze age will still just be bronze age). You already could trade up for your next era goods in advance.
Units of next age might be interesting but I don't see where you got that one from - notably most ages do not have so big a difference that "next-era" units would be a big deal - and they have the significant downside that they can't use AO crit.
Indy->PE is a big difference because of the battlefield scaling - but people are already using OF units in Indy; that PE units in indy might become easier to come across is not game-changing. The same therefore holds for pretty much any era up to OF.
OF->VF->Mars experienced big stat gaps as they attempted to invalidate the hover tank. But even with the stat jump I'm not sure it'll meaningfully change the fighting in the age. Rocket Troops do better than Turturrets mostly because of the change in opposition, not the increase in stats. Ronin Bot will take huge damage to Turturret splash. Maybe mass-ninjas might be impressive in OF but that'd require a lot of troop wins, and if you use them on anything difficult you're going to lose some. Mars Sentinels in VF maybe?
Expanding on this thought:
If the Mass Event Rush was offered as a single item per Event; not a special, not a reoccurring Grand Prize, just once per player, then we can reasonably expect 10 of these every year
Beta Announcement Dates:
Space Age Jupiter Moon: March 2022
Space Age Venus: March 2021
Space Age Asteroid Belt: March 2020
Space Age Mars: April 2019
So we'll probably get Titan sometime around March, and any subsequent Ages/Era's a year apart.
How many full city rushes per year would be considered balanced and not game-breaking?
Only way I really see this being limited in a manner that won't have a cascading effect, is if it's added to something like rewards for reaching a New Age, or the Castle levels. Something that would make it truly a one-time reward that isn't going to keep compounding the longer the game goes on for, and isn't frequent.
If we suppose these are GE5 rewards, then it might be something like 1 every 4 weeks on average (about the rate you currently get certain specific upgrades). Would 13 days production be gamebreaking? We need over 50k goods to finish the age. If we get ~1000 a day from event buildings, that'd be 13000 of the next age's goods covered. A nice perk but I think it'd be fine. Especially if they continue increasing the goods required for a new age