Buffy: S4, fast forward the college years, instead of dropping out season after season, first half light hearted adjusting, enlists shiftless Giles into exploration dark slayer origin that is calling to her, the drifting of core four, Spike gets chipped, Buffy's depression arc with existential realization she can never have a normal life, dropping out, sleeping with a enemy who is a actual enemy and not neutered. Basically have season four be the dramatic season six. Well, it is sleeping with the enemy (very dramatic) its none of the later creepiness. B is a depressed nineteen/twenty year old adjusting to the real world with a real minimum wage job, not like returning from hell, quasi suicidal and abandoning her family and friends etc etc etc its all very dramatic but less dark in effect, because instead of "BTVS the college years" it makes complete sense for S4 to be the major transitory period for the show into the real world. I think fans would be accepting of this plot arc if it came first.
Angel: Going into it, I would have kept the vampire with soul as recovering alcoholic analogy, recast Kate and kept the noir detective set up. Usually in this genre the anti hero detective has someone on the inside providing them with information which is the role Kate was suppose to play. Instead of Kate as cop seduced into drugs during a undercover prostitution sting (her original conception) paralleling Angels' addiction metaphor, I'd have the character possibly be male, well the alcoholic detective is a cliche, it discard the Angel and Kate as a will they or won't they Mulder and Scully parody, having the Scully also be male, counter intitutively works as a subversion, as well as fulfilling the buddy cop traditional set up. Cast Whistler as Doyle. Keep Cordelia as herself, have Faith join the team.