Here's the fallacy of detracting from big names. It doesn't make the smaller names that much likelier, because this detraction isn't going to invalidate all the big names. Let's look at your Heihachi example. He didn't choose Heihachi... so... did he pick Klonoa? KOS-MOS? Valkyrie? No. He went with another big name from Tekken, another face of that series. This is the antithesis of what you want to prove.
It's like saying Sakurai/Smash is unpredictable. That's doesn't help the case for anyone. All it does it make some likelier characters seem less likely.
Ok. Here's how I'll justify them. Because even under the specific criteria that already separates them from a standard inclusion (that you've ignored) in which most of those examples were selected, we got IC, G&W, ROB and Shulk, and not Mr. Stevenson, Diskun, Parabo, and Aeron. Even when Sakurai was specifically seeking historically relevant characters, we got the option representing the Game & Watch, not the Famicom Disk System. The NES, not the Satellaview. Why do think even the surprise characters like DHD, WFT and PP have, despite being surprising, all been very recognizable? Sakurai even said he went with DHD and PP because of how ubiquitous they were.
So even in very contained, sparsely-populated categories, he still went with the more recognizable option. Though it's also disingenuous to highlight an instance of Sakurai, for example, specifically looking for a retro NES character and going, "see!" as if that applies to your standard inclusion. The retro rep spot doesn't even exist anymore anyway. You're going back to Melee for an example like that, when they still included Japan-only characters.
So, let me turn it around and ask this. Who did we get? Shinjiro, Bianca, Tooty, Billy Kane, Hilda, Mechanica, Jecht, Morag, Marduk and Aqua, or Joker, Hero, Banjo, Terry, Byleth, Min Min, Sephiroth, Pyra/Mythra, Kazuya and Sora? Could the alternative, less recognizable list of characters not be made fun? Of course they could. Every character listed could be made fun with enough creativity. But we didn't even get one not-very-recognizable alternative.
If you're going to say it's not because of recognizability, it's because of general popularity, I'd say well... that's tantamount to the same quality being favored, being "big". Or at least, bigger. Either way, that's going to put the same characters at an advantage and the same ones at a disadvantage.
The fact that you're so downplaying recognizability shows how much it's taken for granted. Can you even go through the Ultimate newcomers and isolate one character who isn't highly recognizable, at least from their respective series? It's an incredibly consistent trend, almost unbroken.
: Give him two Final Smashes, Blast-o-Matic when he's winning, and the Kredits if he's in second place or lower
