Define "not very good".
Lost Omens Character guide will also provide aasimar heritage, presumably with ancestry feats. You might consider waiting to see what that looks like.
That's the Advanced Player's Guide actually, I believe.
As for the question, grant something a bonus against a few elemental types, I believe celestials were resistant to acid, cold, and electricity, as well as a weakness to evil damage, attacks that do some good damage, darkvision, and perhaps a fly speed. Bump up the CR by 1 and you should be good to go.
As for how much the resistances and weakness should actually be I'm not sure. I'd recommend checking creatures of a similar level for guidelines, but it seems like it's usually 5 below CR10 and 10 a little above it, with 15 or 20 being reserved for truly high-level foes.
Not good means they just write: give a +1 Bonus to (Everything) AC, Attacks, DCs etc.
I took the example of the Vampire. So giving the resistances Leveldependend. More difficult is how to use Innate Spells and possible Ability boosts. The 1E Half Celestial counted as 2 Levels higher (up to 3), gaining a lot of ability increases.