Findings:
If you survive in early period, late game is too easy.
If you increase difficulty in game settings, the early game become weird (to me).

My Proposed Methodology:
1. Add A.I. Diplomacy term that AI will try to form Coalition against one, if its
supremacy score exceed a certain threshold value.
2. Via quest, create a "boss player" (as late game crisis) (against human player) that dozens of developed new star system and fleets. Trigger by at turn number/ score value...etc.
3. Via AI-only quest, AI players gain progressive permanent boost when certain turns reached.

I am quite new to modding but have tried study it and find:
1. Difficult to ask AI to reform the alliance versus situation changed (Score changed)
2. It seems impossible to create new players in the game.
3. I cant create an AI-only quest.

If you are the pro and interested, please help.