Assassins can be nerfed by increasing board size.
Assassins (in TFT, DAC -and- Underlords) have a weakness to positioning. With proper positioning, they'll waste attacks on stronger units and end up just being pushed into the dirt. Granted assassins can still come back from positioning with lucky crits but even so, they have a distinct disadvantage to it.
TFT lacks this counterplay.
The board is so small that attempting to position a tank in the back line ends up with assassins there, chewing at your adcs. The small board also means the ranged champions end up staying where they are while (as with most assassin players) they leave a "dummy" unit to force the enemy to walk up, freeing up the backline to be eaten alive like a baby covered in raw meat in a lion pit.
With a larger board, you're not only allowing more tanks in the back but helping to negate some of the movement by having ranged units walk up alongside their melee counterparts. It also allows for the assassins to jump back but not necessarily hit the squishy units. This will cause more strategic gameplay as players are now more reliant on skillful positioning than "Me put cho'gath in corner. Me BIG BRAIN. ASSASSIN STRONK." gameplay we're seeing right now.