The current St Pancras is a bit of a mess.
A rebuild could help both EMR and Eurostar
When this was discussed in another thread the concensus was that the capacity of the current Eurostar terminal was around 1800 passengers per hour outgoing assuming the exit checks were properly manned. It seems like passenger capacity through the tunnel could be increased fairly easily, double deck trains, and maybe timing two trains one behind the other to make best use of tunnel paths. Bear in mind the continental end is currently split over a number of destinations, I am assuming the St Pancras limit is currently the governing factor.
The obvious services to move would be the SE ones because that gives you more platforms already connected to HS1, but where do you send the SE service as there are no obvious other terminals convenient to central London without major infrastructure building. But I think the facilities to handle more passengers through the boarding checks is the biggest challenge, there just isnt any more space.
St Pancras has excellent connections so its probably the right place to act as a London gateway to the continent. Going back to or routing some services back to Waterloo is a non starter, the crawl through south London on 3rd rail just doesn't work.
One thought, how much Eurostar traffic originates north of a line between the Wash and the Bristol Channel? Would a second terminal in Birmingham work, using HS2 when its open so that the UK end is split between London and Birmingham.
There are no simple answers to St Pancras, in a way its a victim of it own success and any solution is going to be very expensive, Its a shame that when the redevelopment of the area took place some space wasnt kept clear for future expansion, but that horse has well and truly bolted.
Which brings one back around to the OPs original post. Until the bottlenecks in other places are cleared there is no point in adding capacity to the tunnel, and there are some quick wins that could be made before further tunnels are needed if these bottlenecks are cleared.