Hmmm, I think it would look more odd as a ligature. How would you do it, have the crossbar extending all the way to the second i, or break the crossbar before the first i?

Definitely not the first. Possibly the second. But the best solution could possibly be to use the same structure as above, but move the tittle to the left and move the f-terminus even more to the right.

Looks fine to me.

I think moving the tittle left might make /fii/ in itself look better, but at the same time that rightward straying tittle as-is helps the eye break out of the ligature, which has its own "gravitational force" due to its natural weight. So I think regularizing the alignment might solve one problem but create another. Or at least I'd be on the lookout for that as you experiment.

Try joining the beak and the two tittles with a wavy tilde form.
But I'm not sure you need anything at all. In Romanian there must
be 3-letter (and maybe even 4-letter) combinations that are more
common than "fii" - will you be ligating those? :-) Frequency isn't
the only thing that matters of course; in some designs even the "fi"
should avoid ligation.

hhp

You may be right. I just reacted to the shape when I saw it on a test print, 10/12 points, offset. But I can't make exceptions for everything, I already have a ridiculous amount of ligatures (and that is after throwing out more than half of them) and too many kerning pairs. Thank you, all contributors, for taming my completist lunacy.