Welcome to ethresear.ch Raul 
There are effectively two coinbase addresses, one for the validator and one for the proposer. It makes sense to have the proposer directly receive transaction fees, but there is flexibility as to how we allocate the collation subsidies. We can design the protocol to give collation subsidies in full to the proposer, in full to the validator, or something in between. My initial idea was to give the collation subsidy in full to the proposer to mitigate the proposer griefing the validator by withholding the collation body. Iām now leaning towards giving the collation subsidy in full to the validator for the validator to take responsibility for availability of collation bodies with this scheme.