Let's play a numbers game for a short thought experiment here -
Street Fighter 4 was announced back in early 2008 with a trailer online and an expose' in EGM. Over that spring it got beta tested in Japanese arcades and it made it out that summer as a non-beta arcade release. Home release followed some months later in February of 2009.
Intitial official arcade release had the entire non-super SF2 cast as well as the 4 SF4 newcomers. 16 playable characters in other words(with Akuma hidden as a bonus boss and seth as the standard boss) . A few months later when it came out on ps3/360 we got some SFalpha characters - Rose, Sakura, Gen , Dan as well as Fei Long and Cammy and Gouken were added to the game, seth and Akuma became playable after beating them in arcade mode.
So from July 2008 through to February 2009 , a period of roughly 7 months - 7 characters were added to the game.
After that , a little over a year later in March of 2010 we got Super Street Fighter 4 which added 10 new characters - Dee Jay and T. Hawk filled out the returning SF2 cast, Cody and Guy came back from Final Fight, Adon returned from alpha and then by popular demand we got some SF3 characters with Dudley, Ibuki and Makoto. Rounding out the new additions we received Juri and Hakan. So in about 12 months we got 10 new characters.
At this point the game was supposed to be finished but the arcade version ended up getting an upgrade that added Yun, Yang , Evil Ryu and Oni to the roster. This was intended to remain only in the arcades but by popular demand it made it over to the home consoles in 2011 as a digital upgrade. SF4 was basically finished by this point, the only remaining upgrade it got was arcade edition 2012 which was more about balance fixes than anything else and , as I recall it fixed all versions of the game for free but if you wanted the AE characters you had to pay.
So a quick recap then - from July of 2008 until June 2011 was the main content release phase of SF4. In that time frame we went from 16 playable characters to 39 playable characters. Put another way, in 35 months we ended up getting 23 new characters. Or, a new character every 1.5 months.
Finally, some might say "but Pach, what about Ultra SF4 ? it came out 3 years later doesn't that throw off your numbers quite badly?" and you would be right, it would skew the math quite badly. But I don't count Ultra as part of SF4's main phase of release. It was a bonus release that exists as an effort to rebuild the fanbase in time for SF5, it was a cheap upgrade using assets built for the failed SFXT and lastly, to me - it's in the same boat as something like SFalpha 3 max. A version that exists more as a thank you to fans , a bonus dream match. I expect SF5 will eventually have something similar a couple years after they cease adding content , a PS5 port that has a couple extra characters. Put simply, I don't count it as part of the main intended release schedule for SF4. It's kind of it's own singular package whereas Super and AE were yearly updates.
I think this means a few things for SF5 - first of all , the most recent rumor about there being possibly 3 more characters on disc - that's not at all far fetched. They'll need a boss and possibly a mini-boss as well as a hidden boss or even just a straight up hidden character like Dan.
I don't think we'll get content delivered simply as 1 character a month though, KI has demonstrated that you end up trapped eternally in a bug fixing cycle that makes the game feel like a beta in some ways. Nah, here I think a better way of managing it is about 3 months after launch (so May) they make those 3 NPCs playable with some fight money and they add a single new character via DLC. So 4 new characters all at once. This build of the game receives a balance/bug fix patch in June and that version is then kept the same for EVO. During the EVO top 8 next year , capcom shares a trailer revealing the next 4 characters , maybe partially in silhouette ? Then in Mid October , all 4 release at once with various pricing options (paying zenny , perhaps buying all 4 at once saves you a bit, ala carte costs a bit more and fight money has to be individual purchase). November we get a balance/bug fix making sure the cast (which is now 24 strong) works well for the capcom cup in December. During the capcom cup, capcom teases the next additions to the game. Then , around April 2017 we get 4 more , patch in may , fall ch reveal at evo again , available mid october , capcom cup reveals 2018 spring characters and so on.
The only thing up in the air here is how long they'll keep this release schedule up. Also , I guess how the character additions will be themed - will they make all 4 characters added come from the same game ? will they skip a theme and just make it so out of the 8 characters added over a year , 1 is brand new and the other 7 come from anywhere ? who knows for sure.
I think we'll get at least 3 years worth of DLC for a total of 24 added characters. I feel like , given that 2 are potentially brand new bosses on disc , we will end up with at least 2 more brand new characters beyond them. This leaves 20 potential returning characters through to the end of 2018. Despite all the speculation I think there are a few which are shoe-ins - Sakura , Rose, Juri, Akuma, Guile, Blanka, E.Honda,Dhalsim, Fei Long, Sagat, Balrog , Makoto, Ibuki, Dudley, Elena, Hugo. That means I think we can look forward to at least 6 or 7 characters that I didn't mention that were in SF alpha or SF3, hell maybe even SF4 people too? I'm reasonably confident that we will never see Necro , Remy or Twelve ever again, sorry to their fanbase. I'm not even that sure about Sean and Oro actually. But, that still leaves Urien and Q and Alex ! so keep the dream alive.