She won’t do much in Standard, but there’s a lot she can do in other formats.
For Standard and Limited, she’ll hose the new Spectacle in some cases, but not many. [scryfall]Omniscience[/scryfall], maybe, or sometimes randomly some situations where a player uses [scryfall]Llanowar Elves[/scryfall] to cast something, but that doesn’t matter very often.
There is Convoke.
With the Loxodon, you only have to be careful to pay at least one mana. (There’s also [scryfall]Chord of Calling[/scryfall] for Modern.)
Other formats, well…
No more casting Company with a [scryfall]Noble Hierarch[/scryfall] on turn three. How much will this matter? Sometimes it will be a huge tempo swing, at other times, not so much.
Of course, it’s easy to go under Lavinia with this or any of the many other free cards you might have in Affinity. Well, Affinity isn’t doing very well anyhow, but it might come back.
This might be a bigger deal. Of course, it’s not the only Mox to get hit. Or indeed, there’s [scryfall]Lotus Petal[/scryfall] and [scryfall]Lion’s Eye Diamond[/scryfall] in Legacy.
I guess Tron needs to take this into account when sideboarding. Relying too much on something like Karn can backfire. Tron-lands aren’t of much use here.
Not the Goblin itself, but many things around it. In Storm decks this causes things to cost less, but you can’t cast them without the lands. It hoses rituals as well.
That’s pretty big in Legacy and Vintage.
The other noncreature Delve-cards are already banned, but yeah, this is still out there.
The opposing player must be careful not to discard too many cards, because then they can’t cast this. Breaks up the most broken turns, at least, but you still need to get under these thing for Lavinia to matter.
This has been popular lately in various [scryfall]Arclight Phoenix[/scryfall] decks. I don’t see this happening much early, but there are situations where it might matter. Of course, there are other madness cards as well.
… and it’s ilk.
It’s already out of most formats, but yeah, it still does matter. There is also some other cards with Phyrexian mana, such as [scryfall]Mental Misstep[/scryfall] (Vintage only), [scryfall]Mutagenic Growth[/scryfall] and [scryfall]Noxious Revival[/scryfall].
Also banned in Modern, but possibly matters in Legacy, but probably not.
Even if you discard the cards, the converted mana cost is still what’s on the upper right corner, so discarding six lands means CMC=13.
Cascade is one Lavinia’s victims.
Is this still a deck?
A stand in for Treasures in general. I’ve been casting huge [scryfall]Banefire[/scryfall]s recently with their help.
How about this combo? Of course it doesn’t stop the Spirit, but it does stop the spell it stole.
Random other stuff that can potentially be played in Modern:
All in all, she seems quite random. She hoses things, but only if you can get under them. Lavinia has a lot of uses and can at least slow down quite a few decks, so maybe she’s a good sideboard card, especially in Legacy and Vintage, but I wouldn’t be that sure.
Did I forget something important? Probably. Is Lantern still a thing?