Tear has always been an option, is just that it requires scaling, something that most mages don't want, as their powerspikes are in the Mid Game.Though, once fully stacked, Seraph is a very powerful item, that provides one of the most powerful shields in this game.
I personally don't know if she should have Mana Ratios, as she could become a second version of old Ryze: a champion forced to build 2 Mana items and then builds full tank, who has enough damage to kill a squishy and deals a ton of AoE Damage, while it takes hours to be killed. The huge difference between them both is that Lissandra has a ton of CC, so she would be pretty similar to Tank Ekko in a way.
I like the idea of making her scale infinitely, but not when it leads to unhealthy builds that abuse the newly added ratios.
The only way I could see Mana Scalings be introduced in her kit without breaking her, but have an effect in her kit, would be by adding them only to her Ultimate. It is not spammable, and would enchant her Late Game power of a Teamfight Mage by making it a game changing Ultimate.