Oh wow. I’m in a similar situation, although I’ve had dicey amounts of experience in numerous fighters from the getgo. I ended up buying a few fighters.
Trust me, you won’t learn anything from SF4 if you’ve played SSB that long. I played for nearly the same amount of time as you if not more, and half of that time was the slower Brawl version. You are going to catch hell in sf4. It’s read-heavy, it’s motionless in comparison to Melee, the special moves don’t transition like ssb and you can’t roll, you can’t spot-dodge, jump variation is tipsy, etc.
You’re better off moving to an advanced fighter and taking a slow path from there. And this is coming from someone who did the same. I’m not saying give up on SF4, as you might like it. But the number of differences are humungous.
KOF is somewhat notorious for it’s advanced mechanics. From my point of view it simply has more options. I don’t care for anyone but Kula and maybe Athena, but the movesets I find in the game are rather engrossing. Check out some tournament videos and you’ll see what I mean. You’ll need some fast hands and a PC for the Steam Edition if you want any community though.
Blazblue Chrono Phantasma is the latest iteration of Blazblue, it came out a few years ago but it still has a tournament scene in full. I tried to pick it up one time, mostly offline toying around with Kokonoe and finishing her challenges, fighting the computers (the game has fascinating A.I., which you’ll love), and going through tutorials. This game really does it’s best to teach you, and seriously the A.I. is definitive of what I call a ‘challenge’. Not as in how hard they are(well that too definitely), but how they read what you did last time and implement that knowledge later, making each round unique. BB’s pretty famous for it, actually. Also, their challenge mode helps you to realize the game’s potential. I call this one balanced, but again I didn’t play much online. I like the special moves and characters a lot. As someone who’s been messing with Nintendo’s since youth(likely just as you have?) the character designs are something I find appealing. Their fighting styles are ridiculous, and awesome. Special moves come around quickly but normals are just as important.
I recently got into Arcana Heart 3 Love Max. I wouldn’t recommend it if you aren’t otaku enough (it’s an all-female anime-esque fighter), but the game is fast and really free-moving. The homing attack(I call it flight) is what really sets it apart. The community is scarce, but not as bad as PSN’s KOF by a longshot. Again, this game can get hard if you’re a newbie. Still, it’s a balanced fighter if you know what you’re doing. Trouble is learning to know what you’re doing. lol
I don’t know anything about the other games in this era ( as in, the last Tekken I played was 6 and I was alone on PSP and I didn’t play against anyone but two or three times on PS3), I believe Tekken community will move to Tekken 7 soon, and I have no idea what the Mortal Kombat scene is like nor the DC games or the Soul Calibur games, I would highly suggest you ask and check every nook and cranny!
But speaking honestly, this is a terrible time to be moving into fighters. You’ve got communities disappearing and others getting ready to transition over to something else, and then there’s “vanilla players”. Unless you’re planning on getting a PS4/XB1, your best bet would actually be usf4 in terms of community, which brings me back to the first sentence in this paragraph.
Love Max is the last fighter to come out, actually. It’s region-locked, so people are still looking for other players desperately, making matchmaking somewhat easy, but it also has a “vanilla” version, as in the original Arcana Heart 3, with people being hesitant to get it.