[Taqwus]

The "traditional" abuse in games that allow arbitrary gifting of territory is to give provinces in one AI's sphere of influence to a second AI, in order to start trouble between them. This works especially well if first AI (non-recipient) has some attachment to the province in question. A second form came up in GalCiv where a planet could change sides without war; namely, trading a planet for something while knowing that you'll get the planet back anyway.

Memories are important for diplomacy. Building a rapport over time should matter; e.g. you shouldn't be able to cancel the bad feeling from a recent war with a large gift immediately thereafter. Conversely, they should be readily destroyed given sufficient provocation; the friendlier you seemed beforehand, actually, the greater the perceived magnitude of betrayal. Enemies /expect/ things like hostile magic and assassinations, while doing an "alpha strike" on an ally would demolish that trust completely, provoke vast amounts of resentment, and make it harder to build rapports with anybody else for pretty much... forever, actually. And casus belli should matter as well; a war preceded by well-known provocations on both sides should hit global reputation less.