Thameslink is my local line I am just like how would it work I know it would start in Bedford but like South don't know if St Pancras is good or some where like Blackfriars or London Bridge and what era like to see would be with a 700 and HST we already or would we have new train like a 319 days with Moorgate
I would like it to be the full route tbh. Either bedford to Brighton or Bedford to Rainham (kent) although I'm quite sure we would get neither in tsw. 319 days would be ideal but that means making new trains for the route. So that's out of the window. So class 700 it is.
Bedford to Moorgate would give you a full Thameslink route without going through London core. Or for the south side of the Thames you'd want something around Blackfriars/London Bridge to Brighton.
Given Thameslink (normally) is on the London commuter route (but not on the game of course) I wonder if perhaps Bedford to a suitable route hopping point, such as Three Bridges maybe would be good. So you get MML south, go through London core, and then can hop over to London Commuter. Hopefully at one point DTG adds Thameslink at some point there. For those wondering re suggestions thread vs here - OP is looking to create this route by themselves as far as I'm aware based on a previous thread I've seen.