Looking at much of the list you provided, it seems like strong card advantage might be the main power factor in your cube's environment. As a cube's power level flattens out, having more cards of relatively equal power level is going to trend toward being the best possible option. No longer will it be important to have a specific card; having more cards is simply the best strategy.
I approached this challenge in my own cube in two ways:
- Increase the value and abundance of synergy strategies/decks. This is the only way I have devised to make specific cards matter more without skewing the power curve. It shifts the dynamic away from "more cards is always best".
- Decrease the quality and abundance of card draw/advantage, and spread it across colors as much as possible. For instance, the only draw spell on your list that I run is Treasure Cruise, and it's almost certainly my best pure draw spell. It (and Dig) requires some setup, or a lot of mana, so there are some limitations. I like Cruise more than Dig because Dig gives you too much selection. Brainstorm is great because it is not too powerful, but it is still good, skill testing, and synergistic with the usual Riptide mana base.
You could also just decide to run cards of more varying power levels and it would probably smooth the problem a bit too (though it obviously introduces new challenges).
Since I don't get to draft my cube as often as I would like (once per month on average), I like to test "extreme cases" to explore design concerns I have. One of these concerns was "is card advantage way too good?". I drafted a deck where I prioritized strong card advantage spells over everything else, and ended up in a control deck with quite a bit of removal. It was too good. I made some actually very small tweaks to reduce and spread my draw. I also continued down my on-going path of enhancing synergy options. The next time I tried CardAdvantage.dec, it was good, but far from unbeatable. At this point, card draw spells are good picks, but far from a windmill slam.
Granted every cube is different, but your list of GRBS seems to have a theme.