Yeah I prefer to have resistances you can overcome through mundane means because you researched the enemy, rather than just having a one-size fits all solution.Maybe really, REALLY powerful creatures. Epic tier? No problem with that. Otherwise, c'mon. I mean, you can do stuff like make lycanthropes resist non-silvered, I suppose. Or regenerate from it. Either way.
One solution that would allow to keep the 'signature weapon' trope is to simply have your ability to overcome 'no mundane damage' be simply dependant on you being attune to a magic item, ANY of them. Like, if you have a magic amulet or magic armor or magic cape, you gain a magical aura that just overcomes that basic resistance.
Come to think of it... what if we were to ditch the +X on items and have your +X simply based on how many magical items you are attuned with? The more magic item, the stronger your magical aura? With a few rare magical weapons boosting it further too. Just a thought
4e didn't have level dips. You took a MC feat that gave you a weaker version of that first level and a skill training. If you got the Rogue MC Feat, for exemple, you got to use Sneak Attack once per encounter if I recall. My favorite was the Paladin one because it let you apply the Divine Challenge feature once per encounter. I put it on my Cleric to make him an off tank (story wise he was an old drill sergeant for a Paladin order!) so I could mark a guy once per encounter (also picked up an Encounter power that could mark! I loved that character, he was really fun to play). It was a really cool trick. Further MC feats that let you swap powers were not particularly great and I don't recall people really taking it. I know later we got 'Hybrid Classes' but I never delved into how they work I wasn't interested.Yeah, you could do a three-level mini game before level one (if desired) and keep 1-30 "pure" for actual professional adventurers. Though one of the lessons that 5e learned was to NOT "front load" level one for less multiclassing dips (to various degrees of success). Although I don't recal 4e having much of a problem with that.