I've often wondered about making better use of those goods lines on the MML by introducing a new local service on them, enabling the current slow (Thameslink) lines to run fast through them, in effect removing Thameslink services from the fast lines by making the slow lines fast. If that makes sense.
Primary issues are a) what to do at the southern end, at West Hampstead, and b) at the northern end, at Hendon/Mill Hill.
My preference for the southern end is a new tunnel to Gospel Oak, thence extending the Goblin services up the MML to provide the local service. Interchange with the Northern line at Belsize Park on underground platforms on the link, and with Thameslink at West Hampstead. On the northern end, the minimal case is terminating at Hendon. The slightly better case is extending the goods lines to new platforms on the west side of Mill Hill Broadway. The ideal case is pushing on from there, new tunnels at Elstree, and continuing up to St Albans, which would probably have to move south to accommodate the additional platforms. Net result of that being that Thameslink would be able to run fast from the northern local terminus (Mill Hill Broadway/Borehamwood & Elstree/Radlett/St Albans/etc) to West Hampstead. Running fast means more capacity, and a uniform service pattern on a two-track line means everything could stop at West Hampstead and the northern local terminus. The fast lines would then be free from more intercity services, and the slow lines, with less stops, are then also more suitable for running some of the freight from the Radlett depot over.
...additionally, by separating the local service from Thameslink, you can justify more stations on the route. I can think of several obvious ones off the top of my head, namely Grahame Park, Napsbury, a new station between Cricklewood and West Hampstead, and you'd be able to serve Cricklewood, Hendon and the new Brent Cross quite justifiably.