Are we forgetting that Liverpool Street - Norwich is going 3 tph off peak from 2019? No reason to think that won't include Saturday services as they are often busier than weekday daytime ones. GA have confirmed that this will be an additional service and not the extension of the current Ipswich - London train. So plenty of options there, a fast IC, a semi-fast IC and a stopper seems sensible. That's an additional 500-1100 seats every hour depending on what unit they use (10 car Aventras if the Rail magazine article from last Autumn is to be believed). Ipswich to Colchester will effectively have an extra 2 tph thanks to the Peterborough service extension.
If we're going for speculation, then I suggest the following hourly IC service pattern;
xx00 - Norwich, Ipswich, Colchester, Stratford, LST. With Stratford becoming as important a station as Liverpool Street when Crossrail opens, missing it out should be the exception for a few peak trains, not the rule.
xx20 - Norwich, Diss, Stowmarket, Ipswich, Manningtree, Colchester, Stratford, LST.
xx40 - All stations to Colchester, Witham, Chelmsford, Shenfield, Stratford, LST.
Until we know better, the assumption should be that GA are doing a lot of homework on increasing capacity for everyone, albeit with a completely new service pattern.