Aggregation of tokens

Is your feature request related to a problem? Please describe.
I want to have the best stable coin when I use stable coins. But I don't actually care about which stable coin, any of DAI, USDC, USDT, GUSD, LUSD and UST are the same to me. I would like uniswap to automatically pick the best one as input or output (where the best one = the one that gives me the better price)

Describe the solution you'd like
Allow to create aggregation of multiple coins together, when choosen the routing would just pick the best one in any of these, there is a question to have about how does multiple input tokens are managed ? (does the router is capable to pull and swap 3 different coins in a single route, do you just take the smallest non zero balance, just allow input, ... ?) (and also propose a few agregation by default, like the big 3 DAI, USDC, USDT and BTC WBTC, renBTC).

Describe alternatives you've considered
Checking all the tokens one by one to see which is the best.

Additional context
It would be cool if aggregation would allows to support custom ratio, so you could say that DAI is worth 0.99$, USDC 1$, USDT 1.01$ or even much more radical changes.
I would understand if this would be reserved as an expert feature, this doesn't sounds like a straight forward idea.

No files, tests, or entry points are named. Begin by clarifying how token aggregation should choose among inputs and outputs, handle multiple balances, and apply custom ratios; done requires an agreed design for routing and default aggregations.