I want the old bosses added back before they are done (except maybe Shiden). Then all the old movesets will be covered and this really can be considered an "Ultimate" version.
Since DLC characters have no story, there is no reason not to include them (especially since Ein is already in, so being dead shouldn't stop any of the bosses). Besides, all the dead bosses were associated with a company known for making bunches of clones, so bringing them back would pose no trouble at all if you needed a story reason.
Tengu is at the top of the list. Out of the six fighting games in the franchise (1-5 & Dimensions), he's been playable in half of them so it's a shame he's left out of this one. He also has the most unique fighting style of the bosses that are left out, and he already has 3 different costume designs (in addition to his first costume also being available in a few different colors). Plus they could add a 4th costume design where his nose is missing and he's got katana wounds as a joke about how Hayabusa was supposed to have killed him and Gen Fu took his nose. Plus he should totally get a Santa suit like everyone else.
Raidou is next. He's been playable in 1/3 of the DoA fighting games, and is pretty important lore-wise. His fighting style borrows from several people at once, so he's not just a "clone character" like some people consider him to be. He also already has 3 costume designs (the first of which having several color variations...though to be fair his third costume is just his first costume with regular colored skin and no aura).
Genra comes in third because he's only been playable once (though he did appear in another game as the final boss) and his moveset needs some reworking. I think his projectiles can be made to work (several characters can teleport after all), they just need to be tweaked some (maybe make them only usable from a certain stance, like his weapon is in Dimensions). The projectiles are kind of iconic to him, so I wouldn't want them entirely removed, but if need be they could just be incorporated into his throw and counter animations so you still see him do them. Unfortunately he only has a single costume design (which came in 3 different colors) unless you give him his pre-Omega costume from the Ayane cutscene.
Shiden is last of the four because he's only been in one game where he was a secret character you could only access after unlocking everyone else, and his moveset is the least unique as it is just a variation of Hayate. Plus Hayate is already wearing his costume as his C4, and Shiden only had that one costume design (though it came in 3 colors). I'm not opposed to him being added--as I really don't mind "clone characters" as long as there is some variation between them (ie. Bayman/Leon)--but he definitely has the lowest priority out of the group.
Along with Shiden, there are a few other characters that would probably be variations of characters already in the game but that I wouldn't mind seeing added, but consider them low priority. These include Hitomi's Father, Chen (Brad Wong's master), and Jo Hayabusa (Ryu's dad). I'd also consider an un-armored Nicole (as a variation of Rachel) in this group, as--like Argentus--I did like her personality and the idea of a 6'8 Filipino fighter is pretty good.
I'm not really interested in Donovan getting added, because it seems they are saving him for final boss material in a later game and it would be kind of weird for him to get added as a story-less DLC character to this game.
As for Miyako and Niki, we haven't been directly shown that they are fighters at all. I'm not opposed to them getting in, I just place them lower than the other groups. Of the two, I'd prefer Niki as at least she was shown going into dangerous situations with Zack (ie. living skeletons). We also already know Niki's deal, while we have no real idea what Miyako is up to. Besides, if we were going to get one of Fame's mistresses, I think I'd prefer Isabella since we don't really have much story on most of them we might as well get one with a more interesting visual design (and at least Isabella isn't Japanese or American).
I'm not really interested in seeing any brand new characters get in, as they have virtually no story (and I've already seen people making up all kinds of stuff for Marie Rose since they don't know much of her actual story) and there are still existing characters waiting in the wings. As long as the three main bosses get in, I'm not completely against brand new characters getting in as well, but I would prefer any of the above characters (except Donovan) to them. I really think brand new concepts should be saved for DoA6 though.
The only exception to that would be Irene & Rio since they're pretty heavily requested and we already know who they are, plus both have appeared in the franchise before and we know they can both fight. However, since they would both probably need completely new movesets, I could also see them being held back to be a big attraction for DoA6.
I'm not interested in any more guest characters at all. Especially as paid DLC. If they absolutely must add another Virtua Fighter character, then based on the series history I think Kage-Maru deserves the slot, but would really prefer no more guests whatsoever.
Overall I'd probably break it down like this:
1. Tengu
2. Raidou
3. Genra
4. Irene / Rio
5. Shiden / Nicole / Hitomi's Dad/ Chen / Jo Hayabusa / etc. (ie. "Clone characters")
6. Niki (maybe switch her with the above)
7. Miyako
8. Brand New Characters
9. Donovan
10. Guest Characters
If this was for what we wanted for DoA6, I'd definitely rank New Characters higher.