I'm new to tsw so not entirely sure how or if this could work but why not instead of adding expansions to lines they're merged with others. Here are my suggestions for this: Glasgow to Edinburgh + Fife Circle line This would of course interchange at Waverly and allow people to travel from Glasgow to Edinburgh then onto the fife line aswell as adding additional layers to services out of there too. This would open the door for further expansions out of Edinburgh like a northern ECML potentially to Newcastle? ECML + MML This would be the more exciting one for me, merging these 2 would add so many possibilities for routes and would add excitement and detail to both of them. This would be done by adding the link line between Nottingham and Newark (Castle and Northgate) which could then continue to Lincoln to mirror the EMR service between the 2 cities. LNER services can also start at Lincoln going down towards King's Cross. I'd add the class 222 for EMR which can be used between Lincoln and Leicester via Newark and Nottingham.
Merges can't happen unfortunately, extensions are technically possible but fat chance of one. The two routes you've suggested, however, don't really need to be connected (although an ECML extension south/north to York or MML to Sheffield would be nice).
