Tseng (Final Fantasy VII) - I mean sure but you already fight the Turks quite a lot and Tseng himself isn't really all that hateable of a character. He's just a man doing his job, so I don't lose any sleep over not getting to fight him.
Maester Mika (Final Fantasy X) - I agree with wanting to fight him but I do have to address something. He didn't Send himself to avoid seeing the world after the Yevon deception was unveiled, but because he genuinely believed there was no hope with the Final Aeon gone and that Yuna and company had just doomed Spira.
Also you technically do get to fight Mika in X-2 as one of the Via Infinito bosses, though that's not much of a consolation, much like with the next entry.
Jihl Nabat (Final Fantasy XIII) - Absolutely no argument here. Even worse than Mika because there's pretty much a perfect place where such a fight could have happened. Simply have Jihl head down to the engine room after Sazh and Vanille escape the brig, that way the pair get their own boss fight in Chapter 9 before reuniting with the party.
Jihl's inevitable failure to defeat them there would give a much better reason for Barthandelus to kill her and indeed would tie into his subsequent line about human weakness, ie "Without our help, death is all of which you're capable".
Emperor Gestahl (Final Fantasy VI) - Haven't played the game myself but even from your description, I'd say it's arguably more satisfying that he's betrayed and murdered by one of his most heinous crimes that he had used as nothing but a tool. At least I'd say that's what the thought behind it was.
Queen Brahne (Final Fantasy IX) - Similar idea honestly, the karma of her being killed not only by her previous benefactor whose ideas she'd used to gain power but also by the Eidolon she had summoned to defeat him seems satisfying enough.