I think it would be awesome to have the opportunity to buy livery packs as DLC (licences permitting) for trains we currently have. To have a developer/team working on livery art in order to sell it would be a great money making idea for DTG (possibly other 3rd party developers too). I appreciate that we have livery designer but personally I find that I don't always have the time or the nack to make great representations of liveries used on various train types. I would happily buy livery packs as they would vastly increase the visual array available to use across our routes.
I would get some BNSF or ATSF locos. The GP38-2 (BNSF), SD40-2 (ATSF; BNSF), and AC4400CW (BNSF) would be the only engines available at this time, but maybe others could be added later.
As the porting of preservation locos from TSW2020 to TSW2 for livery editor requires remodelling of the locos to implement on TSW2, to incentivise the process for Dovetail and and encourage being added to the roadmap, Dovetail could consider creating skin packs to be sold as DLC for the locos ported over, if there are no licensing issues.