I've always thought that my favourite fights in D&D were those where two evenly matched adventuring parties, both with a variety of skills and talents, squared off. They can be very challenging, the fact that they increase in power every time you meet shows that you're not the only person in the world progressing in power and finally they're more interesting than a lone final boss because they almost always have more versatility.
My suggestion is thus: introduce a single-encounter adventuring party in BG:EE, who then return twice in BG2:EE and once in ToB.
This is a mod I've been entertaining for a very long time, but I'm too bad at modding with DLTCEP or NearInfinity to create it. I am totally happy to write dialogue for the four encounters however. I appreciate that this is a fraction of the mod and anyone who implements this will be taking on the brunt of the effort. Hopefully there's a skilled modder out there who loves rival adventuring parties enough to go for it!
A rough sketch of the adventuring party as of BG:EE:
Name: Ormund Greyblade
Male Neutral Evil Human Fighter
Biography: The only child of a prominent and wealthy noblemen from Calimshan, Ormund's ambition and greed compelled him to attempt a coup against his father. He failed, and was forced to flee in to preserve his life. He became a mercenary in order to make ends meet, but dreams of the day he can retake his birthright and claim vengeance.
Female Chaotic Neutral Half-elf Blade
Biography: Whatever Mirth hides behind that white mask, it's unlikely anyone will either will ever see it and live to tell the tale. She is reticent with details of her past or even her true name, so her allies named her for the dry laugh she responds with when queried. She met Ormund when attempting to rob him, but he managed to convince her to join him with the reasoning that a party of mercenaries/bandits would be more effective than a lone wolf.
Female Neutral Evil Gnome Illusionist
Biography: A cheerful young gnome, Darlyassa's affability masks her utter lack of morals and violent nature. Her past varies with the telling-and the listener-but one can always discern a deep-seated distaste for her family and the idyllic town in which she was raised.
Male Lawful Evil Half-orc Fighter/Thief multiclass
Biography: Violent and brutal as he may be, Frukul's loyalty and affability to Onmund are unquestionable. He has no deep trauma or shadowed mystery in his past-he simply enjoys the thrill of battle and the feel of gold in his palms-and for a job as lucrative and unique as killing a demigod, there's certainly a lot of both up for grabs.
Female Chaotic Evil Dwarven Cleric of Cyric
Biography: An agent of Cyric, Halda's uppity demeanour obscures an incredibly perceptive and strong-willed mind. She and Urlagh met Ormund's group as they set up camp to lay in wait for the Bhaalspawn in Cloakwood. Argumentative and arrogant, the reason she travels with an Elf of such a different temperament remains unknown.
Male True Neutral Elven Fighter/Mage multiclass
Biography: The most out-of-place member of the party, Urlagh is polite to a fault and almost unnaturally eager to please. He is especially subservient to Halda, who treats him with a measure of respect lacking in her interactions with others. Despite this affability and gentleness, Urlagh has no compunctions whatsoever about killing those he is instructed to kill.
Sorry for the huge wall of text, hopefully someone takes interest in the idea.