One thing I like in a set of wargaming rules is the inclusion of some element of randomness that takes the certainty out of a players turn - whether this is diced movement as in the Lardies games or the Blunder! roll in Black Powder. Browsing through the Fighting Fantasy Battles volume of the old 1st edition Forces of Fantasy supplement for Warhammer I found the following optional rule that dealt with terrain and hidden monsters.
The idea was that whenever troops entered a piece of terrain for the 1st time in a game they ran the risk of encountering hidden monsters. A percentile roll was made on the following table and the resulting monster placed on the table presumably under the control of the GM although in the absence of one I'm sure it could be controlled by the opposing player. This roll was only made on the first occasion that troops entered the terrain.
I think it would certainly add a bit of fun to certain games, perhaps not scenarios but certainly encounter type battles. The table could easily be modified to whatever random giant animals/ monsters are in your collection.