Gimmicks aren't what veteran players have grown accustomed to, or rather, what's kept them clung to this game. Maybe that's the reason behind why certain character-exclusive abilities, such as Squirtle's Shellshifting and Lucario's OHC have had mixed feedback ?_? I wouldn't consider giving "depth" to Brawl characters being gimmicky, however. A gimmick implies that
only impractical and / or limited ideas (i.e, unusual) are being used in an innovative manner in order to give a character more of a reason to be used than others, which isn't the case for a fair selection of characters. Zamus, Wolf, and Ike haven't aren't unworkable nor do they heavily rely on their buffs; all of the aforementioned characters simply received a lot more depth than their vBrawl counterparts had. I don't consider Ike's Quick Draw getting the buffs that it did, nor Zamus's hooligan shenanigans as odd and unusual to the Melee feel. They don't detract from or slow down the gameplay much, from what I can see, and offer both characters a wider array of options to mess around with and hopefully develop into various playstyles. There are more examples for characters that can plop comfortably right into Melee's groove, but for the sake of time, I'm just going to address the atypical characters (attributes).
Lucario's OHC system helps Lucario keep up with the undoubtedly and significantly faster pace of the Melee environment, without giving him a stupid-crazy attribute directly comparable to Fox's waveshine Usmash. It readies the upcoming metagame for a notable, but far from overwhelming threat, despite there being a myriad of options that Lucario can choose from. Pit and Squitle have gained a considerable boost in viability as well with their respective side-B's -- another attribute that gives them the ability to compete with the rest of the cast -- along with multiple other changes. If those changes are supposedly gimmicky, then what about the veteran cast getting buffed to compete with top tiers? Bowser:M, by that logic, is a gimmick; any Melee character getting a buff that gives them greater viability should be considered gimmicky as well. If that's the case, why should anyone code or play Project:M when Melee is obviously superior?
Some characters cannot be played like the typical smash characters, granted, but it's not as if they're immutable when played against as if they were just another smash character. Any character that's considered "one big gimmick" can get ***** by typical spacie stuff just as badly as the next character. It's not as if said 'gimmicky' characters won't be able to compete either. They have redeeming factors about them that add quite a bit of depth and creativity to their respective metagames. If a character receives less popularity due to an odd learning curve, then so be it. There's nothing overwhelming about them and every player has the choice of picking them up or not. Also, removing / challenging their key abilities and traits wouldn't completely shut them down -- I'm very sure that if that were the case, the idea behind said ability would be scrapped before the public becomes accustomed to it.
Just tossing stuff out, feel free to attack this theory-filled post