Flattered that you’d ask! Couple of factors:
how often do you get / are you likely to get Fated Summon draws? If you’re close to one right now, whether you wait depends on whether you think you’d like more than one or two of the next list.
Speaking of which, you can see the next list (albeit, still tentative, not guaranteed!) over at the FS master thread (scroll down to 4] March 04th 2024 ) and the predicted heroes all have links.
At a glance:
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blue You have some really nice blue 5*. Pairing them with elemental-defense-down blue (Frida, current set, King Arthur, predicted in next set) is tempting, but frankly I think also balancing your colors a little more is probably best for general use
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green Your Hemidall is an old-school but still useful overhealer with a modest chance of revive. Chances are at some point you’ll really want Mother North (predicted in new set) — heck, I still don’t have her myself. Even though Heimdall and MN overlap, having a high-% reviver is a really nice option — if you aren’t going to have many Fated Summon picks, Mother is one to prioritize.
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Margaret (current list) has dodge-scales-with-damage effect which is… an interesting option, and it’s often nice to have some Very Fast hero options in your mix.
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red You already have a couple of pretty decent reds; in the current list, Baldur does self-heal plus autohit plus the sneaky useful utility of resisting mana drain; that latter is actually pretty useful given the number of mana cutters around. Next list has Santa, who is old school but not shabby.
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purple A standout in the predicted coming list is Guardian Panther, even without her costume. If you only get two picks in the coming Fated Summon, it’s likely to be her and Mother North… and if you aren’t sure how many picks, and especially if it’s likely only going to be one or two, then that might be a reason to hang on until next list.
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yellow Tempting to pick a yellow if you can, given that you don’t currently have as many 5* options outside uncostumed S1. Bai Yeong (current) gives you a tactical “interrupt” option at Fast on offense, and Guinevere (upcoming) gives you an old-school mana cutter (along Cookie, whom you’ve got) who also throws heal-over-time.
But a lot of it is how many picks you think you’ll have based on your playing/summoning history…
…and a useful tiebreaker is checking out the classes of the prospective picks vs how many emblems you currently have. (To a certain extent, which ascension materials as well.) This game tends to be a “long game” (unless you want to spend pretty significantly), so “what you have stuff for already” shouldn’t be the only factor, but it’s worth keeping in mind — it’s a bummer to level a new pick only to have to wait again for mats and/or emblems.