I watched both of these movies recently and saw quite a few parallels which make me think there may have been some intentional borrowing. Both Destroyah and Megaguiras include:
1. The search for the perfect Godzilla annihilation weapon
2. A scene in which Godzilla is set upon by dozens of mini versions of his monster enemy
3. A showdown between Godzilla and either an aggregate form or energized form of the mini monsters
4. The set in which Godzilla has his final showdown with Megaguiras was also used in Destroyah
5. Both end with Godzilla eliminated
6. Both hint Godzilla will be back
These are a few similarities I noticed with a none-too-careful viewing. There may be other similarities.
Your thoughts?