Valve has provided more information (opens in new tab) on the "Big Update" coming to Dota Underlords in the near future. Along with another selection of heroes drawn from the Dota 2 roster, the game will be adding a system to keep the hero pool from overflowing. Specifically, it'll be throwing some of them in jail.
Here's how (and why) it works. As the hero pool expands, it becomes harder and harder to rank up the heroes you've purchased from the shop, since the odds of finding matches for heroes on your bench get reduced each time a new hero is added to the general pool. Valve says it doesn't plan on removing any of the heroes that have appeared in the beta season, and so when season one kicks in and adds 12 new heroes to the pool, some of them will get tossed in jail for 24 hours.
Each day, a different group of 8-12 heroes will be jailed, effectively banning them from appearing in the shop. This won't be completely random: Valve says that the list of jailed heroes for each day will be created such that players can still complete every alliance.
A nice added benefit of this system is that it prevents an established meta from taking hold and making the game feel stale. You're going to be faced with different composition options each day.
Valve also revealed (opens in new tab) the rest of the heroes who will be appearing in the Big Update. They include Shadow Demon (a tier one heartless demon), Bristleback (a tier four brawny savage), Sven (the ace of knights, who is also human and scaled), Io (a tier three primordial druid), and Faceless Void (primordial and the ace of assassins).
There's still no word on precisely when the Big Update will drop, but judging from the way Valve's talking about it, it'll be sooner rather than later.