There is the potential to add more platforms to the MML section of St. Pancras by building out over the road. Might only manage 2-3 platforms, but every little helps.
Personally, I don't have a problem with Thameslink being longer-distance per se, but it needs to match distance-wise on both sides. Brighton to Bedford does match at about 60 minutes to St Pancras/Blackfriars from each direction. Again, on the northern half it's then just West Hampstead until Mill Hill (limited 8 car stops at Kentish town, Cricklewood and Hendon notwithstanding), whereas on the southern side it's just London Bridge until East Croydon. Beyond there I'd prefer a cleaner demarcation of services, but Thameslink is limited by the fact it's not on a 6-track route, so beyond Mill Hill to St. Albans it has to serve double-duty as a metro. That said, inner suburban services between St Albans and Luton are fine, and outer suburban services from Luton to Bedford are fine. The southern side is a bit better as the metro services between East Croydon to Purley/Redhill are handled by Southern, but likewise, the inner Suburban services from Redhill to Haywards Heath and outer suburban services from Brighton to Haywards Heath all match up fairly well.
In an ideal world the MML freight lines would be extended north from Mill Hill to St Albans and south to somewhere like the LO network and then Thameslink could just solely focus on suburban duty. They would then just need two type of stock: a higher-density layout with lots of luggage storage for the inner suburban (airport serving) Luton-Haywards Heath service, and a lower density layout for the Bedford-Brighton outer suburban service. The MML has East Midlands services for fast services, and Brighton has the East and West Coastway services via a reversal as well as the fast services it justifies all on it's own.