Aftermarket Engine ECU and the 7G Auto?

[260DET]

A possible project 2000's car idea with a problem to solve. To simplify engine tuning and get rid of all the dash light nuisances the Merc ECU's functions, except for the transmission, would be done instead by an aftermarket ECU. But I guess that to work the transmission certain engine related inputs would be required and that's were I come to a full stop.

What I'd like to have are opinions on the practicality of such a project and how it might be done.