I don't really want to get into this discussion, but I've noticed that you could play O5E and A5E at the same time (say, with an O5E character using exclusively O5E rules and an A5E character using all the A5E rules) with the DM/Narrator having to do no extra work whatsoever. (Beyond knowing both sets of rules). I don't even think it would be at all confusing.
I might do this with any player who is resistant to learning new rules. "Fine, don't bother then, you'll keep playing O5E".
That alone makes it pretty compatible.
Yeah, that is the one way I could see it working. Players can make D&D characters or Level Up characters, the D&D players function off of D&D rules, spells, feats, magic items, and so on, and the Level Up characters use the Level Up versions. Monsters work mostly the same way, but if a D&D character is hit with a critical only the monster damage dice are multiplied where if a Level Up character is hit by a critical the static modifiers also multiply, and so so.
I dunno man, at my table one player has an PHB Champion and rocks PHB feats and another has a retooled their Clockwork Soul to have the LU sorcerer base class with the Clockwork soul sub class. This is at level ten and we haven’t had any problems yet.
Right, D&D characters having D&D feats and other rules still seems to work. That might be the best way to add Level Up to a D&D game, using the two rule sets side by side essentially.
I’d give up. He hasn’t played it, but he insists it can’t be done despite the fact that people are actually doing it. Don’t tempt another essay!
There's a perfectly workable way it could be done, just have two different rule sets for two different character types. That way the D&D characters don't lose out on the D&D versions of things like Fireball, Counterspell, Bag of Holding, Polearm Master, Great Weapon Master, Sharpshooter, Expertise as double proficiency, and so on, and the Level Up characters use the Level Up versions and the Level Up class content, ie, Expertise Dice, replacement feats and different core rules, crits double static modifiers, etc. That addresses most of the incompatibility issues by side-stepping them. Whether it's smooth and balanced requires testing.
That was what my earlier line of questioning was about, in fact, trying to get more specifics on the "play testing" that went on before Level Up's launch. I didn't get any specifics. "We play tested it" didn't tell me
how it was play tested. With D&D rules, or Level Up rules, or a mixture as some have proposed above? With basic unoptimized feat-free, multiclass free simplistic builds, or optimized builds with feats and MC? I still don't have an answer to any of those questions, so to the best of my knowledge those playtests are happening right now, and haven't in the past. "We play tested it" tells me nothing about how it was play tested.