So Senna is a sup and Lucian is an ADC...

Prob not, I think that is just a unique feature for rakan and xayah

they must have some interaction, maybe Senna can spell shield him from greater range than other champs or they both get +25 attack range when near each other... something like that

25 Attack Range would be absolutely ridiculous.... Cool idea, but it would be horribly OP.

However, I assume she has some kind of ability that interacts specifically with Lucian. I hope they do not give it a carbon copy rip off of Xayah/Rakan though. Uniqueness is good in games.

i would imagine they get like empowered q or r would make sense*[slayer-pantheon-rainbows]*

One of the things that makes Xayah and Rakan unique is their relationship, which is (quite intentionally) fantasised a fair bit, hence why they get certain empowered interactions with each other. Lucian and Senna don't have such a "fantasised" relationship, rather their's is more realistic and, more importantly, not as concrete due to the fact they've been separated for a considerable time.

I think them having unique interactions with each other (possibly including unique recall animation) is fair, and quite expected, but it doesn't make so much thematic sense for them to have empowered interactions.