it's a silly thing to post about, but it's come to mind. I've thought about this stuff far longer than any person ought to. Undertale is not good for the imagination, and does not bring restful dreams at night.
anyway, I've long guessed that if any single Undertale character could figure out a way to slip out through the Barrier with a minimum of fuss—possibly without the need for outlandish physics or weird magicks or screwing around with SOULs—it'd be Flowey. because Flowey's a plant, and therefore has the option of reproducing vegetatively. Flowey could set seed, even. we know the "Golden Flower" (whatever species it is) produces irksome seeds.
but can a seed pass through the Barrier? Fox sketches the Barrier with only a few lines, so there's not much to go on. physical objects can wash in, but what can leave? it's not textually clear whether the Barrier obstructs all matter. possibly it's a barrier only to living beings (without a sufficiently powerful SOUL) in which case...maybe a piece of matter could be smuggled through. Flowey's very patient, and plants can live a very long time.
heck it might have already happened in some fanfic or other—my knowledge of Undertale fan-writing is practically nil (for the moment) and "Flowey gets a seed past the Barrier" seems just about obvious enough for someone to have explored already.
~Chara