Side b is great but of course you already know that and it’s predictable. I like to short hop fast fall fair out of the opponents range, a lot of characters have a hard time dealing with that (nair is good for this too when they’re closer to you). I use a lot of dtilt out of dash too especially when they’re shielding. If you hard read a shield shieldbreaker can get you a free kill in neutral. Other than that I do a lot of dash dancing and short/full hopping feint a lot of attacks out of them. It keeps your opponent off balance in neutral
Lucina doesn’t really have a super diverse neutral so it’s all about movement and baiting and punishing and mixing up your approaches. Use a lot of tilts and jabs out of dash too and if you see an opportunity you can definitely land a few fsmashes in neutral out of dash