I know this is for character addons, but I feel the need to talk about more than just that...
First off, if you're gonna be playing a lot of the game's base levels with custom characters or other non-level content, I'd recommend
Custom Savedata, which lets you save single player progress with custom content loaded. Just add your addons first, and then load this. It uses entirely different save data though, so you still cant get emblems, unless you do it with this file. I also only recommend doing this after you've beaten the main game with no addons loaded, so you don't spoil the game for yourself. I know you said "No WADs", but SOCs may be fine.
For more levels, I'd say that the
Official Level Design Contests and
Unofficial Level Design Collab are great collections of community made level content! I'd also say to play
Sol Sestantia - If you've ever heard of the "[Sonic game] in X Minutes" series, this is a pack by the same author, and it really feels like exploring one of his animations! Finally, for an oldie but goodie, there's
The Emerald Isles- its certainly rough and aged, but if you wanted to know what good level design looked like in the old days, this pack's still fun!
As for actual
characters,
Neo Sonic is my favorite of the "alternative" Sonics. It carries the most important elements of Modern Sonic without the need for a ton of other functions the game isn't made for (sorry Modern Sonic), while looking super fresh to boot! Shadow isn't available yet, but he will be soon, and he's a blast to play, but he's for more experienced players.
Metal Knuckles & Tails Doll are a combo pack, with Metal Knuckles combining Metal Sonic and Knuckles traits together, and Tails Doll being a fluffy jiggly lad who's really cute. Oh, and there's also
that odd statue that jumps weird and shoots razor checker boards. Whatever
that's about.