Yeah should be fine surely! Buying Joker doesn't add him to the game, it's basically just a method of unlocking - he's already there, so they could defo use him.
Other puzzles: thinking broad brush here, perhaps a multiple choice "trap" where Layton sets three buttons and can put them in different arrangements, pressing the button again triggers them in the same vein as Snake's C4. I'd have that as a neutral B where you can hold the button and then press a direction to pick a different button to place or trigger (like Shulk's Monado Arts wheel - and you could probably make it work so that you can rapidly press B to choose which to place/detonate as well). And of course I'm still a big proponent of the moving blocks!
Can likely draw inspiration from trunk minigames too. Don't ask me which trunk minigames though... never cared for them much myself!
Another Spirit idea: imagine a spirit battle with all the various characters from S Mystere. Lady Daliah with her cat as Rosalina & Luma, Flora as Daisy, Stachenscarfe as Mario, and no doubt R.O.B. will feature in some form too. I'd get excited for that! I'd say Don Paolo as Wario, but I don't think he has a good skin for Don Paolo in this game. Maybe Doctor Mario, in his purple alt, would be an appropriate stand-in?
You'd think that, but apparently you can't even save any online replay's if it includes any DLC characters you haven't purchased.
then again, apparently the Persona Spirits are still obtainable regardless if you purchased Joker so perhaps we don't have to worry much about that. Unless the spirits that include Joker himself or his stage are left out, I'll probably have to check up with someone who hasn't purchased him to see if they still managed to get all the Persona Spirits regardless.
that sounds interesting, if they can get the multiple choice puzzles implemented would certainly be interesting to see.
As far as the Trunk minigames go, I don't know, if we where to ever get a second Layton character, I always envisioned the animal buddies working better with Luke's moveset but I suppose the animal buddies can work with Layton too, one popular idea I've seen around, is the hamster from the second game, where Layton throws an apple and the Hamster goes after it, if the opponent picks up the apple or is in the way, the hamster will attack them (Like the Frog and Snake forms do from Prince Sable's assist trophy)
If Waluigi was playable, he'd be a perfect stand in for Don Paolo, but I guess he can be included as a Hostile Assist Trophy" But then again, Dr. Wily would probably fit much better as a hostile assist trophy for Don.
As for spirits for other Layton characters.
or
for Luke with a
helper
for Flora with maybe a green
at the beginning with maybe some
helpers
for Emmy that mainly relies on roundhouse kicks as well as the fatal frame chick as the hostile assist trophy
for Descole
if they can't include other DLC characters in spirit battles
as Stachenscarfen, would probably spam his up special in reference to his affinity for hint coins.
As Inspector grosky, he deals damage while running and will also continuously use his dash attack as well as any other belly based moves. He also runs extra fast.
for Sherl
for Granny Riddleton, with maybe a red
as puzzlette, a very tiny
for beasley and a small purple
for Keats
for Aurora at the Umbra clock tower
Not sure who Katrielle and Alfendi could stand in for,
I suppose
for Katrielle since their both pretty snarky and have a servant that is basically their personal whipping boys but take it in just fine...
for Alfendi maybe? they both have very long noses and can lose their temper fairly quickly? Not sure.