When Senna released, there was a lot of disappointment that it didn't really make sense to pair her with Lucian. Demacia still hasn't got most of its cards this expansion, but people all assume this will just be Poppy's Demacia side (I agree that it likely is). I thought I'd come up with some cards that would both make sense to pair Senna with Lucian, while also being Poppy support.
Apologizes, but no art. The focus here is mechanics, and I feel a crappy image I find online would just take away from that.

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The biggest thing that made sense as something to synergize with both Senna and Poppy, is they both have Attack effects. This gives you access to Attack effects even when you do not have the attack token. I think can be a fairly powerful effect, especially with Poppy, but isn't main deck-able, so onto the cards that create it:

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Comparable to Cursed Keeper, the power of this card is in its last breath effect, but you can't block with it, so it is hard to kill it. Possessed Commoner is even a little more difficult to kill than Cursed Keeper, as it has Spell Shield, the opponent can play things like Make it rain with worrying about killing it.
Cursed Keeper is often used killed using Shadow Isles cards, and if you are running Senna, this would likely be how you obtain the Lone Undead (also helping you level Senna/Lucian). If you are not running SI, Demacia still has a decent number of ways killing your own units, through challenger and "single combat" effects. Of course, it is difficult to get a whole lot of value out of Single Combat with a 1 power unit, but when paired with Poppy (who of course greatly benefits from the Lone Undead), you can buff the Possessed Commoner up before playing something like Single Combat. Additionally, due to having spell shield protecting it, Possessed Commoner is a great target for fast and slow spells that require an ally to resolve (Glimpse Beyond, Crumble and Single Combat would be the most common examples) and many of these spells can be ran with Senna or Poppy.

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Petricite Smith works well with fast damage and kill spells (Senna uses a lot of these), as they can allow you to kill a unit's blocker after blockers have been committed.
It also works well with double attack, giving double attack units value out of their second attack if they kill their blocker with the first. Lucian is a classic example of a double attack unit and Pompous Cavalier (which is supposed to be Poppy support?) has it innately.

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Isles Ferry Helmsman acts as a decent target for Lone Undead in basically all situations. It provides extra incentive to kill its blocker while Petricite Smith is in play (to keep it alive). Also should act as a decent unit to play with Poppy, as it can be buffed by her and provides draw so you can play more cheaper units to buff as well.

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This is to provide extra incentive to Senna decks to kill their own units; a strategy that Lucian is a natural fit in.
Using Lone Undead on Undead Mass allows you to start a "free attack". Killing a Possessed Commoner with a Darkness then targeting this acts a pseudo way of dealing 3 damage to the enemy nexus, as if they block and kill the Possessed Commoner, you get another Lone Undead to summon it again. If a scout unit(such as Valor) is the weakest unit that died this round, it can cause a rally to gain the attack token.

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This is primarily Poppy support, allowing you to flood the board with small units after you buff up the Poro Platoon. I could also see this having uses in Lucian decks, just to give him more allies to see die so he can level.
I thought about the poro tag here, but felt like this would be way too strong with Poro Snak. I think this card is reasonable for a Poro deck as above. You can still buff it with something like Sharp Sighted on the round it dies to summon multiple Plucky Poros, and those can all be buffed by Poro Snaks.