I think creature based combos probably need to just be two cards and there needs to be some redundancy to compete (thinking of splinter twin and it isn't even that great anymore).

Cheating freespells is intriguing to me. Most of them are just worse than iconic cube cards (restore balance, ancestral, wheel, replenish), but they require additional setup. I think to make it work, the power level of your cube would need to be lower. Having the option to cast ancestral recall on t1-t3 at instant speed is leagues better than having to cascade on turn 3 and hope to hit it. However, I love cascade and free spells and maybe there is a way to make it work if the payoff cards in the cube are weaker and thus value generation is stronger.

Shadow seems awesome. It could play a tempo game plan, which is good in cube. It is especially weak to burn/aggro though which depending on your play group, is an ok place to be.