The Future
People are discussing whether they have enough coins to upgrade as soon as the last TF finishes upgrading; and some people have talked about materials.
Before the upgrade, you needed two commodities to upgrade: time and coins. If you played a lot, time was the scarce commodity. The bottleneck was the time taken to upgrade the last character.
Now, there are more commodities in play: time, coins, materials, super rareium. Upgrading from level 10 to 11, the scarce commodity seems to be either coins or time. The materials are not a problem.
But in the future, it seems you need an extra super rareium for each upgrade above level 10 - hence one for 10 to 11; two for level 11 to 12 - and presumably five for level 14 to 15. The super rareiums only seem to come from the hardest mission portal - for missions of 7 hours or (probably) more. This makes me think that at the highest levels, the logjam will be accumulating enough super rareium - the coins, time and other materials will all be there once the super rareium has been gathered.