I agree with ensemble and projection design. I also agree with the separation of Play/Musical score and playwright as a separate award from Best Play.
As to best song and replacement/successor - I think these categories should be treated with different voting processes similar to Revival categories when the number of nominees is uncertain. Allow nominators to vote for 5 songs/successors (regardless of gender, billing, or show type) and if a consensus is found in the results include the category. I believe they would need to keep the number of nominees in the category loose - 3,4,5, or none depending on where the voting leans - at least while the categories are being established. Though with any category I think it's always better to have a smaller group of deserving nominees than a larger group with tacked on also rans.
I know there are, or at least were at one time, provisions in the Tony guidelines for giving awards to replacement performers but I believe they suffer from 2 problems. Because it isn't a regularly awarded category it isn't top of mind for voters so even if they see someone worthy they don't think about awarding them come Tony season. Secondly, previous threads discussing the few times they seemed close to giving a Tony to replacements were turns by star performers. While this is certainly worthy of an award, if it were to become an actual category I hope some lesser known talents would make it into consideration. Even being on the same list as Bernadette Peters for ALNM or Jane Lynch for Annie, could give a working actor the bump they need to take their career to the next level.
Writing about previous threads on the topic has reminded me of the biggest argument against a regular best replacement category which is how do you get nominators to see them all? It's easy when Bernadette Peters is returning to Broadway but will they really see every Elphaba and Glinda? Does the strategy then become inviting them to see whichever performer producers feel gives them the greatest edge? It's a tricky issue but an interesting one that I feel could be used to everyone's advantage. Tony's get another category- a performance based one (which let's be honest that's what John Q Public is interested in). Long running shows get a way to boost interest. Performers have more chances to further their credibility/prestige.