In the rare case where a legendary creature becomes a copy of another legendary creature with greater mana value (for example, if Sakashima the Impostor is a copy of a legendary creature with mana value 5 or greater), then when that creature dies, if it's still a legendary creature card in your graveyard, Havi's last ability will be able to target it
Finizen
The trick I figured out to do this was to count how many you'd already cast (okay, technically cards in your graveyard, but most likely they got there by you casting themcounting cards in the graveyard was easier than remembering how many you had previously cast) and incrementally increase the damage for each card
Im not seeing it taking the place of another card in another deck without the support of the reduction or upside of planeswalker much though
Thankfully, Bloomburrows Peace Offering precon has a number of viable win cons across its decklist that are well worth exploring and are even furthered by some slight upgrades
Here's a gallery of some of my favorite doctored cards