Yes and no. A5e classes are not more powerful than, say, WotC classes, so mechanically there shouldn't be much issue in terms of balance. That being said, A5e classes do have more options, so it may feel a better fit if you're converting to look at existing A5e classes with an eye toward toward providing more choices per level, maybe even integrating some A5e innovations like combat maneuvers if appropriate.
I will say, keep your eyes peeled for even more a5e classes coming out that will fit those niches. However, in the meantime, a5r currently has 30 classes (+2 more in the next few months) between 1st and 3rd party (+7 motif classes, which are special combo classes that go up through level 8). a5e is nothing if not full of options.
As Micah Sweet said, a5e classes are not more powerful, they just have more options: especially outside of combat. While adding combat maneuvers to existing 5e classes is one way to give them more options, my opinion is that adding knacks (each class has a different name for these features, but collectively they are known as knacks) to them will make your players feel like they are getting the full experience.
If I do stick with 5e, combat maneuvers for martial classes (and even, to a much more limited degree, some caster classes) would be something I would look at including. But Broken Synapse makes a good point as well about social options. I've looked through A5E a little today and, while the jury is still out on if I'm a full convert (though I can certainly see myself playing a game or two in the system because it looks tight as heck), I can certainly see myself incorporating more social abilities (both active and passive) into my own games.
When thinking about it more, the idea I had was to take a look at 5e classes (both WOTC and Homebrew) and quantify how many skills each class has for combat vs. social vs. exploration and use that as a metric to decide what they all should get. In my mind I'm currently favoring a slot-based approach (again, assuming I stick with 5e as a base, which I'm not fully committed to just yet) wherein classes like a Fighter, for example, who doesn't have as many social abilities would have, say, double their proficiency bonus in "social skill slots" to use and then a menu of skills to fill those slots. Passive abilities might take up one slot, but active abilities might take up two or even three.
Admittedly I haven't given this much thought or really crunched the numbers yet outside of what is a very surface level idea. But I like the vibe of it and might look into it further.
Another thing about A5E that I enjoyed quite a bit was the way classes had multiple feature options at certain levels, allowing for additional customization on the part of the player. I don't know if that's something I'd be eager to homebrew in for 5e, but I do like that as a concept and it looks like A5E really pulls it off well without becoming overwhelming for the player (choice paralysis and all that).
A5e Classes were designed to meet all three pillars found within an adventure. Combat, Exploration and Social Interaction.
A5e also offers up a lot of customization for your character's origin by splitting up race/species into heritage/culture for a slightly more RL feel.
I am a fan of doing the heritage/culture split over race/species for multiple reasons, but certainly because it gives a more true-to-life feeling. Sadly, that's not as relevant to the type of campaign I'm planning since it's going to be heavily inspired by the likes of
Wayward Children, a book series by Seanan McGuire featuring kids who are whisked off to other worlds (in the vein of Narnia, Oz, Wonderland, etc.).
My idea was to have the players all make their characters siblings who are in their early to mid 20s. The setting would be one inspired pre-great war Victorian/Edwardian era, and I could see it being that a number of the characters are either in the midst of or just finishing up their university schooling when they are invited by their uncle to stay for an indeterminant length of time at his countryside manor. It is there that they discover that not only does he have world hidden in his pantry or what have you, but there's also one in the billiards room, the dining hall, the library, etc. The players are therefore playing this "fish out of water" style for most of it, which it quite a draw for my group. I'll give them a limited amount of time to spend in each world and when they come out there's a "cooldown" and also dice rolled to determine how much time passes in their absence.
There will, of course, also be much to do outside of the various worlds, but that is all for another thread.
In any case, thank you for adding another vote for the A5E! It's certainly another system that I am curious to try out and play with some more.
From what you describe here, it's sounds like 5e or a variant like A5e really is what you want. It's the best I can think of that hits all these well.
I would think that Pathfinder 1e or a 3.5e game might be worth looking at, but it sounds like you already know that will mean tossing Bounded Accuracy out the window. They would do a good job of checking your other boxes, though.
Yeah the bounded accuracy is a big draw for me, which is why I have not stuck with either Pathfinder (although I understand PF2e has a variant that is like bounded accuracy) or 3.5e. Nevertheless, there is a lot to learn from both systems and I certainly think they do a lot of great things!
