I'll just come out and say right now that saying the game'll be bad based off the current 10-Pokémon roster is... just stupid. I mean first of all we practically know for sure this is not gonna be the final roster; I kind of doubt they'll show off the whole thing before release, unlockable characters are a thing in fighting games, and due to how low-key and cloney Pikachu-Libre's reveal was I sort of don't think that'll be the last reveal. Second off, even if somehow we only end up with 10-15 characters... I kind of think you're missing the point of a fighting game here. I mean of course, variety is fantastic and the more characters the better; but a lot of beloved fighting game franchises like say Smash and Street Fighter had their first game's roster be incredibly meager. Did that stop them from being great games? No! of course it didn't! I mean, hell, in Smash 4 I play like 10 characters - Shulk, Lucina, Ganondorf, Mewtwo, Link and a few others - and that doesn't stop it from being a great game. Why? Because the quality isn't directly equivalent to roster size. What matters is how the game plays and what other features we have. Smash 64 for instance had 8 starting characters, 4 unlockable ones, 9 stages, VS mode, target smash, training mode and board the platforms. And I know, that's a Nintendo 64 game and this is a Wii U one. But despite the meager roster size, that is more than enough content to keep me going. There's so many fighting games where people just pick up one or two characters and use them the entire time. As long as the roster has variety and quality, quantity shouldn't be that much of a problem.
And in general dooming a game before it's released based on limited information and pre-conceptions is just generally stupid as well. Remember the hate Wind Waker got before it's release? Hell, remember how people on Smogon thought Mega Evolutions were going to be a one-note gimmick that'd only give like a 10% increase to some stats or something? Sorry to get so passionate about this, it's just... this looks like one of the best things to happen to the franchise in a long while, with a fun varied roster; people who have played the arcade version saying it plays great; a lot of clear effort and passion put into the graphics and music; and a team working on it well-renowned for putting out some of the best fighting games in existence. Fuck, doesn't thinking a game's going to be bad based on what limited information we have fall under baseless speculation? C'mon guys :P
If you want a better example of a wildly acclaimed fighter with a limited roster one I immediately think of is Skullgirls. That game started with 8 characters and 6 DLC ones later for a total of 14 characters. But still what made Skullgirls good was that it was fast-paced, each character felt unique, and the overall mechanics felt tightly made.
I think the reason people are disappointed with the roster is due to two things:
1. There are over 700 Pokemon with dozens who you could imagine being great fighters. Expectations often misguides judgement as people start comparing how the game actually is to the game they created in their own head which can often lead to disappointment. This also includes character rosters and surely with the character roster as big as Pokemon they'll include as many Pokemon as possible, right? Well, yes and no. They'll include as many they think is necessary and would be interesting, but they won't throw in a Pokemon just for the sake of having that Pokemon in the game. Going back to my Skullgirls example, in a fighter you want each one to feel unique and with Pokemon while there's potential for a lot of Pokemon to have a unique fighting style there's also probably just as many who would fight similarly to another (not to mention in fighting games there are certain tropes that people expect to be filled). So in truth the Pokemon we've seen so far are probably what the Tekken team and the Pokemon Company felt best represented certain fighting styles. Also let's not forget about the Assist Pokemon which helps stack on the number of Pokemon even though they're essentially just one use items, but hey how else would they have been able to add in prevos (Pikachu is the sereis mascot he gets special treatment) and some Pokemon who just aren't suited for this type of fighting game.
2. The most recent Tekken game was Tekken 7 which had 27 characters (though only 20 are playable with the rest the other 7 being added as DLC). Tekken games in general are known for their large cast of characters so people who know the Tekken series probably expected more or less of the same thing. However there's a major difference between making a fighting game you created and a fighting game using another franchise properties. First off you got to work with the people who own that franchise so right away there might be conflicting opinions. Each group may want to include certain Pokemon for various reasons, the franchise owner may not feel the game creators have gotten the character quite right, the game creators are having a difficult time working with a character the franchise owners want in, etc.. Secondly the Tekken team are able to have so many characters in Tekken because they created them, they know these how those characters act and fight. Now while for the most part you don't need to worry about a Pokemon's personality since they're animals, you still need to understand how they fight. Think of it this way, would it look right if Machamp was a fast Pokemon who's fighting style was quick boxing jabs? How about if Pikachu or Weavile being slow but they do large sweeping moves that do tons of damage? Or how about Gardevoir doing martial arts? No, it wouldn't look right at all (it'll look funny though, but that's not what you want). You want the characters to feel natural, like how you would expect them to fight. Pokemon haven't really ever fought like this in a game before so this is new territory and to do it right they'd need to do a lot of brain storming, discussions, and research. I wouldn't be surprised if they have read Pokemon manga and anime episodes featuring the playable fighters to get an understanding of how the Pokemon creators see these Pokemon acting and, most importantly, fighting.
We need to take this game as it is and not what we imagined it to be. We can have expectations, but they should be reasonable expectations and ones that come from understanding why certain decisions were made. I'm sure they'll also be introducing more characters anyway, heck more will probably come via DLC which we know was confirmed. Also let's not overlook the characters we have now and their playstyle, by Types and body shape they're quite varied even though there's only 10 (Pikachu Libre not withstanding, but hey every game has its clone characters. Pit and Dark Pit anyone?).
And who knows, if this works out maybe more we'll be seeing more varied Pokemon games. In another thread I think Vader_the_White commented that even though Pokemon are said to be able to do various things such things aren't usually focused on besides them fighting. Now while he was only talking about within the main game series that comment and this read reminded me of an idea direction that Pokemon could take. When it was first announced I thought Pokken would mainly be filled with Fighting-types (and it should, I know they want to make it diverse which they done a good job with but personally I think it kind of unfair to Fighting-types. I think the game should at least be a fourth or third filled with Fighting-types. At the moment I think they'd have no problem finding a place for Hitmonchan (boxer), Hitmontop (capoeira), Hariyama (sumo), Medicham (meditation & yoga), Pangoro (gansta), Hawlucha (lucha Libre), Heracross (attacks with its horn), Gallade (swordsman), Chesnaught (armored bruiser)) so I thought of what types of games would fit other types. Here is what I thought of:
Firefighting: You create a team of Pokemon who work to put out fires, perform rescues in burning buildings, etc.. While it would include mainly Water-types it'll have Pokemon of other types too depending on the job they'd need to do. For example Fire-types would be useful as rescuers and maybe dealing with forest fires by burning down path of trees to prevent the fire from spreading.
Forest Creator: Here me out. Basically think a cross of Sim City and Ages of Empire. You basically create your own forest for your Pokemon to inhabit and grow. Not only would you be growing and expanding your territory but you'd also have to protect it from rival groups. I imagine it having mainly Grass- and Bug-types but honestly any Pokemon who lives in a forest area would be in. Heck, maybe even expand from forests and make include other environments like mountains, caves, deserts, volcanic, arctic, etc..
Racing: I first imagined it for Flying-types as air races but thinking about it you can have all sorts of races with all sorts of Pokemon participating in them. Flying races, swimming races, digging races, and of course running races.
That's the main ones I thought about before shoving it aside to think about later (which I never did), but maybe that was a good thing as with Pokken including Pokemon of other type it dawned on me I could easily do the same thing. As it turns out Pokemon are very versatile creatures. ;)