How do you feel about bespoke systems versus House Systems versus "universal/generic" systems. Are there certain types of games where you prefer one over the other? What about the proliferation of one system, be it something like 5E or something like PbtA, that eats up a lot of design space in the hobby? When you decide you want to run something, how do you decide on the system?
I love the idea of generic/universal systems, but over the course of playing a lot of them, in the '90s, it became extremely clear that none of them are what they say they are, and it's a big problem. Like Champions/HERO is a good system, but it's basically squad-level combat system, that works really well for small-scale back-alley fights or small groups shooting at each other (whether with guns, superpowers, bows, magic, whatever), and it's not really "generic". GURPS is very much not "generic" or "universal" either - it's a quite gritty system that's really good at doing a certainly kind of Hollywood thriller, like, say Jurassic Park and might be okay for something like Die Hard, just barely - but really feels completely out of its element in a lot of other situations, particularly any involving significant supernatural powers, or storytelling conventions which aren't Hollywood thriller from the '80s or '90s. It doesn't even work well for, say, martial arts movies from that era.
I could go on and on. But the point is, generic/universal systems aren't. They all have something they're actually good at, and work in sympathy with, and a ton of stuff they make really weird. Some people might enjoy that weird, but it is weird. SWADE, for example, true to its name, is pretty great at pulp-y action, somewhat adjacent to GURPS, but better able to handle magic and futuristic tech. It's no accident that it's become the Rifts system replacement of choice for most people.
I do like Cortex Prime's "build-a-game" approach. I used it to build a Mass Effect RPG, looking at the various FATE, 5E and other designs people already had out there, and I was honestly happier with what I got built than I was with those others by some significant margin. It was quite a bit of effort, but still probably trivial next to designing a TT RPG from first principles. I ran it a few times and the group really liked it - unfortunately we have a dyed in the wool Mass Effect hater who is one of the most reliably available players for that group so can only play it when he isn't there, which means we haven't for quite a while. There are limits to what Cortex can do - I don't think you could easily make a particularly convincing "modern realistic tactical" kind of game using it very easily, for example.
I think there's a big difference between PtbA and 5E so it's odd to me to see them equated re: design space. PtbA is a loose set of systems and a mental approach to when and why you roll dice that has produced a wide variety of games that play extremely differently. Whereas 5E-based stuff is generally just literally D&D 5E with different "skins" on, and sometimes not even that. I think the latter is much worse for the industry, and I think the d20 Boom-Bust demonstrated why that is very well - because you get a lot of companies investing a lot of effort in products that can't stand on their own, and the moment the game they're linked to begins to falter, they all collapse too. I don't think 5E has been as bad, not even nearly, as d20 was here, because I don't think 5E has "crowded out" other RPGs to the same extent - but I do think this is a bit of an issue, and we're seeing some of the same depressing (to me) design with 5E-based games that we did with d20 stuff.
d20 Modern, I know it has its fans, everything does, is kind of the nadir of game design in a lot of ways, to me. It claimed to do all these genres and styles of gaming, and it was terrible for all of them. Lovely art. Lovely presentation. Fun, modern-feeling rulebook. Terrible game. Just terrible. This is exactly what I don't want to see - a system that doesn't do well with a certain genre, being used for it, just because people are somewhat more familiar with the basic mechanic of the system. I've literally never been more disappointed by a TT RPG. I expected them to do clever stuff and they did nothing clever at all. Most of the d20 attempts to do similar games, like Spycraft, just really didn't work for me or my group. You can't play like you're competent trained operatives it says you are in the text of the game, if the rules just completely don't support that, and instead make you a bunch of bumbling goons who make the Archer team look like peerless pros until you're like, level 7 or more.
So I guess what it comes down to for me is, I want rules that I believe support the intended experience of the game. I like rules that also aesthetically/conceptually please me, but if the rules work well for the experience, I don't mind ones I find a bit ugly. Like, I don't like d100s. Sorry. At all. But does CoC work for CoC? Yes it does. This means if I believe the rules line up with the experience, I'll happily use supposedly-generic ones - like SWADE for Rifts, or HERO for a street-level superhero campaign about biffing villains in alleys.
I do love a good set of extremely precision-made and perfect rules though - one that springs to mind is MASKS, a PtbA-based "young supers" RPG, which just wow, the main rules are so good for the generating the exact kind of action it describes. Like you do what they say, with even mildly cooperative players? You will get the result they describe. It's a beautiful thing.
I'd also say, just because the rules are made for a specific game, doesn't mean they work for it, and I'm staring at Exalted 1E when I say this, particularly. Don't get me wrong. I loved Exalted 1E, it was a huge breath of fresh air. I think its existence inspired a lot of people, and probably helped keep fantasy going as an RPG genre when it was declining and becoming an older-player thing. But goddamn those rules sucked to use, and very often did not absolutely NOT produce the results that were apparently intended, especially as they were extremely complicated. So you have to assess on a case-by-case basis.
I'm also exceptionally lucky in that my main group of players is very good at picking up new rules and whilst they groan, much more willing to than it sounds like most groups are, so we can use the right tool for the job without causing havoc.
