Tierney has been really injury prone so far, so I think we should go for a reliable, steady player to split time with him rather than a raw youngster, who might have a really high ceiling, but would likely also cost us points if Tierney ends up being out for a prolonged amount of time. Can't think of any names though to be honest.
Speaking of fullbacks though, I've seen quite a few Wolfsburg games this year, they're looking to challenge for a CL spot in the Bundesliga and one of their best players has been Ridle Baku, who started the season as a RB, but is now playing at right wing and performing very well there as well. Would love him as Bellerin's replacement.