As for the concept of reworking NPs, changing their effects seem highly unlikely by this point, but ignoring that for the sake of discussion, it would make sense for the effect to match the lore shared in the CE if possible. Of course, matching the lore of the servant is important, but focusing on the CE itself helps to avoid stepping into the realm of what skill/NP strengthenings do as well as CCs.
For Jalter, the CE doesn’t mention anything about her being a dragon (and being a dragon witch can simply mean you control dragons, it doesn’t always mean that the witch is a dragon, if she was then she’d already have the trait like Artoria). But the burn idea makes sense for the lore of the card, it mentions that her existence is due to the flames, so it makes sense for her to have a burn or even an ignite effect like that one Foreigner has on her bond CE. Not sure about the added crit strength, she has a skill for that.
For Mordred, her card features her questioning her identity due to her dual nature as an heir and a betrayer, along with a mention that putting on her armor allows her to escape both identities. So either you could base it on her armor (though her third skill is already based on her armor, so not sure about that) or some sort of effect that portrays her dual nature I guess? Would have to think about that one some more before I can suggest anything.
(after thinking a bit): If based on her armor, one of the things it does is obscure her identity entirely like with Zerkerlot’s armor, so perhaps an effect like being immune to NP damage debuffs until the first time she uses her NP that battle? With an added defensive effect I guess since that’s niche.
As for the other angle (the dual nature thing I mentioned), maybe a charisma effect and a self star gather? Portrays her kingly side and her treacherous side somewhat.
For Chloe, I’m not completely sure on her lore. Do others feel her pain as well, or does the mark simply make her feel other’s pain without the reverse? Because how she talked about it in the CE made it seem like it was one-sided and she’d just be feeling her master’s pain.
Looked it up, and it seems it was a specific thing for her and Illya, but they brought it over to FGO due to the parallel of Illya being a ‘master’ of sorts over Chloe apparently. But perhaps she knows the curse well enough that she can replicate it onto her teammates for the effect you described. The face card thing is over the top though, maybe she could keep the crit buff from her current bond CE or something like that.
Unfortunately I don’t have a translation of Summer BB’s bond CE, so I can’t really discuss that one.