I don't know if it has already been suggested...

What do you think of a possible limit to number of move orders (attack/transfer) each turn? Maybe a fixed number like 10 moves per turn or a function of number of territories/armies.

The main benefit would be to limit the "steamroll effect" once one empire has reached a certain size. Smaller empires are more focused and dont need so many moves.
It would also add another level of strategy, not just where to move but also which moves to select.
It would probably also make turns for large empires a little faster.

The effect is similar to a turn limit (which mostly effect large empires) but without the stress of giving many orders in a short time.

Possible bad effects are that it would probably take more turns until the game finishes and it might seem unfair to large empires.

I could imagine a turn limit being quite intriguing for a multi-attack game. However, I think number of max move orders/turn should in some way relate to either army size or number of territories held by a player or both, otherwise it would just frustrate players and/or just prolong games unnecessarily.
For multi-attack games, some of way to limit the depth of attacks would be even more useful, like: Allow multi attacks, but only 3 territories deep.