Seems I was quite lucky; all of those mechanics have a place in my cube! Other than Flashback, they were a bit lacking in options too. Populate especially; there's some good multicolor options, like
Growing Ranks and maybe
Vitu-Ghazi Guildmage, but there's almost nothing in monocolor.
To weigh in on morph, I would never run it without having at least 10 or so instances of the mechanic. But my players always find it cool when it becomes a minigame (having 3+ morph cards and the opponent being faced with a wide array of possibilities even in game 3). I also have a couple unique reasons to run it. With the lack of manafixing from land sources, morph helps to alleviate manascrew and provide an incentive to splash. The floor of a 3cc 2/2 is still weak, but in my environment, a colorless card that can become something powerful at instant speed cuts it, and contributes a lot to game texture. So I would consider it a well-designed mechanic, although the 2/2 floor is often hard to scale to modern sets, making the value of morphs today much more tied up in their unmorphed version than 20 years ago in Onslaught block. Power creep sucks, but the more powerful morphs are still great in a low-powered environment.
I also totally support changing the P/T or cc of morphs if needed. I think the biggest reason they are often excluded in cubes is not so much their floor (this is RTL after all, if people liked morph enough they would errata the hell out of it), but that they are difficult to tie in to other themes. Paying mana to flip a creature upside-right doesn't naturally synergize with much in the way that, say, Dredge meshes with Flashback. So people see a few isolated morph cards that they sort of like, and that ends up not being enough incentive to give morph the density necessary for the mechanic to work properly. This all makes sense, but it's a shame that morph isn't explored much for these reasons.