I would love to see an RFC-style standard "multiple-colored-coin-protocol" written by reps from all of the major protocols and that meta-merges the features of these implementations

We actually tried to do that in 2014-2015, but that effort have failed...
Nobody was really interested in collaboration, each company only cared about it's own product.
Especially Colu, they asked everyone for requirements, and then developed a new protocol completely on their own without taking anyone's input.

I'm not sure that merging the protocols makes sense, as some protocols value simplicity, and a combined protocol cannot have this feature.

I don't think there is much interest in a merged colored coin protocol now.
Colu is moving away from colored coins, as far as I can tell.
CoinSpark is now doing MultiChain closed-source private blockchain.
CoinPrism also seems to be largely disinterested in colored coins.

We (ChromaWay) won't mind replacing EPOBC with something better, our software could always support multiple different kernels so adding a new protocol isn't a big deal for us.

So if somebody is interested in a new protocol please ping me.

One of ideas I have is to decouple input-output mapping/multiplexing from coloring.
So one layer will describe a mapping, e.g. "Inputs 0 and 1 should go into outputs 0, 1 and 2".
In this case it will be possible to create more advanced protocols (e.g. with support for 'smart contracts' and whatnot) while also keeping them compatible with old ones to some extent, e.g. a wallet can safely engage in p2ptrade or CoinJoin transactions without understanding all protocols used in a transaction.
 
- in collaboration with feedback from core developers that understand the direction the protocol will be taking and the issues to avoid.  

Core developers generally dislike things like colored coins, so I doubt they are going to help.

Blockstream is developing a sidechain with user-defined assets, so I guess they see it as the preferred way of doing things:
https://elementsproject.org/elements/asset-issuance/
 
As it stands, investors have to install multiple wallets to deal with these varying implementations.

Actually this can be solved without making a new "merged protocol": one can just implement a wallet which supports multiple protocols.