Here are some ideas for making best use of vehicles featured in existing and potential future DLCs: 1). Class 380 for Cathcart Circle. These share the same sounds and similar bodyshell/interior fittings as the Class 350 and 444/450. Possible to layer in to a future Edinburgh to Glasgow route or other electrified Scotrail routes. 2). North London Line + Class 378. Class 378 being part of the electrostar family having similarities to the Class 375/377. Class 378 can layer onto Bakerloo Line. LO Class 313/1 as another potential unit. Something like a Freightliner Class 66 + FSA/FTA wagons for freight (modified from ECW 66). 3). WCML South London Euston to Milton Keynes. Class 350 made using parts of a potential Class 380 from Cathcart. Class 378 + 390 layering onto Bakerloo, 1972 stock appearing on DC lines in return. Class 377/2 (a Class 377/4 like on BML/ECW but with a pantograph). 4). Route out of London Waterloo (to e.g. Reading/Guildford/Basingstoke) Class 444/450 modified again from Class 380. These could then appear as AI on Brighton Main Line at Clapham Junction. 5). Mk3 bodyshell Multiple Unit Such as a Class 150/2 for West Cornwall Local Possible reuse for a Class 318/320 for Cathcart Circle Class 455 for BML/Waterloo route. Class 321 for a GEML route (alongside a Class 315 similar to the 314). Class 319 for BML or a new route like London to Bedford or Liverpool to Manchester Victoria. 6). ECML South + Northern City Line Could feature a Class 365 (similar to the Class 465) Or Class 387/1 (modified from Class 387/2 on BML) Class 313/0 (original as built Class 313) East Coast HST (modified from the GWE HST) Class 91 + Mk4 coaches (only thing that would have to be completely new). 7). Another Southeastern route Reuse of the Class 375/465 with maybe different subclasses Such as Charing Cross to Tonbridge, or Cannon Street to Gravesend. Or as a backdated route with the Class 33, Class 465 (as built) and a new Class 411/423. Class 376 could feature as another add-on also being similar to the Class 375/377/378. 8). Another Scottish (longer distance) route Could feature the refurbished Scotrail HSTs in Inter7City livery with power operated sliding doors. These are made up of former GWR HSTs so the same could happen in TSW using the GWE HST. Would be appropriate on popular route suggestions like Edinburgh to Glasgow via Falkirk and Edinburgh to Dundee via Kirkcaldy alongside a new Class 385 and Class 170. Add to that a Class 37 (from TVL) for a possible West Highland Line Extension. And a Class 143 (for TVL) being reused on an Exeter-area (Riviera Line) or Welsh Valley line route. Please note I'm not saying these should all be made, but are options for a few potential future add-ons that might make better use of existing resources (only if it actually helps save development time and makes content easier to develop instead of building all vehicles from scratch).
Not sure on some of your "similar to" there. Same body walls and ceiling fixtures maybe... As an example class 376 and 377... Different doors, front and rear ends, different seats, passenger bays in doors, no tables, toilets.... Even the window reveals are a different shape (may well be the windows are different too) So in essence this means rebuilding the interior almost entirely (bar the ceiling and the light clusters) and also the exterior and both ends... That sounds like a lot of work
The 380 shell has more in common with a 700 than the 450 - it was the intermediate step between the Mk1 Desiro UK and the 2nd Gen Desiro City
It does look slightly different to the Class 350/444/450 in that it has a slightly less boxy shell, but the seats and traction motors (sounds) are the same I think.
How about Calder Valley Line Manchester Victoria Station to Leeds City via Halifax that uses West Cornwall Local Pacers and Class 150s 158s. New Train is Class 195 Northern rail. That way you don't have to rely on NTP Manchester Victoria to Leeds City via Huddersfield.
i have had similar thought like "how could dovetail make the most of existing Loco dlc" someone here created a well done mod where it adds the MP15DC to Sandpatch and it got me thinking about dovetail can add such thing to the game what i came up with uses similar principles to Preserved Collection where you own 2 things and you unlock this, why not apply it there you have the MP15DC and Sandpatch here is a download that adds the MP15DC on Sandpatch, this can avoid the common stuff dovetail says about doing stuff like that. like "we cant guarantee players will have both dlcs to do that. and no im not talking about layering