Also, Cloud's stage would be awesome but it has the problem that PS2 and Kalos League have, which is the whole some stage hazards are ok but some are incredibly dumb for comp play. But unless something with the angels is actually problematic we should have that Bayonetta stage fully legal, if not a CP maybe a starter when it comes out.
From the OP
Before I answer I guess I should admit my answers are mostly from a perspective of that counterpick stages should be based on giving the player a "true" advantage. Obviously not to the significant level of an auto-loss on games 2/3/4//5, but enough to where there is a point of where the stage can slightly shift a competive set between two players and make the selection of stages that are often dismissed for "jank" to be used to punish players or regions who reject useable stages because of lack of familiarity or complaints. Even then, at the highest level, counterpicking stages should be a more prevalent strategy to actively show the thought process that goes into the stage decision at higher play where the stage matters more. Current 7 stage stagelist isn't good for this because 4 stages are similar, even if blast zones/designs are slightly different, and the others are just as similar except for the tree/floating platform. This makes bans to some point (the only benefit to the small stagelist is 1 ban) pointless because there are so many stages that are similar it doesn't actively change things.
This isn't a reason for 2 bans in the 7 stage list (because it makes BO5's impossible), but a reason why a larger variety of stages would be beneficial.
(TL;DR- actual stage advantage or strategic benefit is dying because we're getting too conservative with stagelists. We saw this hurta game because a playstyle that was valid, but the player base ******* about it grow strong because viable stages were removed.)
Should Halberd be considered legal for tournament play?
Short version, yes. At the bare minimum it's a stage with a direct different setup from other stages. It has some characters it directly benefits and does not maintain the similarity that makes bans irrelevant. People will complain about the blastzones, but that is a direct benefit to certain characters who kill vertically. Only complaints otherwise come from the hazards. All of the hazards on Haliberd are incredibly predictable and once there's enough game time it is not difficult to avoid. We shouldn't call **** jank because either the opponent used the stage better than you (aka they got a setup into the bomb/laser/claw), or that you didn't know the stage enough to deal with those hazards. I think the benefits of the CP value for the stage, especially since the hazards are on a timer and very telegraphed makes it a decent CP stage.
Should Delfino Plaza be considered legal for tournament play?
Yes, same reasoning as Haliberd, it's a good CP stage, arguably far more beneficial for more characters than any of the other stages mentioned, has no hazards, just changes in scenery. Low ceiling is a benefit to certain characters, and so are some of the walkoff parts would cause concern for camping. This stage is less of a threat than it was in brawl because CG's don't exist anymore. But since there are literally no actual hazards (except possibly the water) there isn't a problem. the general design is relatively predictable.
Should Castle Siege be considered legal for tournament play?
Out of the stages listed, this one is the one that i'm least sure about because of the second transformation. The first and third transformations are perfectly ok for competitive play. Either way there are direct benefits to certain characters, it is distinctly different from other stages and CG's don't exist, making the walkoff porton of the stage not as polarizing as in earlier games. Either way the second transformation is only once per 1:35, (if the timing of 45 seconds per transformation didn't change from brawl) means you only have to deal with this transformation 2-3 times at most, depending on matchup.
Should Wuhu Island be considered legal for doubles? Possibly singles?
Yes, glitches have been fixed, certain problematic stage designs get resolved because no CG's. Yes they could promote camping, but that style is inevitable based on character options or playstyle, is large enough to support doubles and different enough to have a benefit for some characters.
Assuming five stage strike, should Lylat Cruise or Dreamland 64 be used as the 5th stage
Honestly, the more I think about stagelists, I feel like Lylat should be the 5th stage. Neither creates a huge problem in terms of being a CP stage, legal, etc. But in a world of where the stagelist of legal stages is rapidly dropping (thank goodness that we'll have Umbra Clock Tower in February) I think Lylat's better for at least better options in the original stage selection just because of it's difference. As it stands, if you go with the most conservative stagelists, we have the triplatform stages (BF/Miiverse, DL, T&C) and the flat stages (FD, DH, SV). While Lylat is similar to the other 3 platform stages that are commonly legal, it's different enough to not only give an option for characters who don't like the setup of DL/BF/Miiverse or the SV/FD division. it's different enough to where it gives characters who do meh on the more common triplatform stages a better option.