Yes, certainly old marketing literature showed the possibility of all sorts of things for the MRTT. The 1997 'Pocket Guide to Airbus Military Derivatives' included versions with a fuel capacity of 77.5 tonne, including 5 ACTs and a MTOW of 164 tonne.
It also showed AEW&C and 'Government and VIP' versions being offered, as well as tanker versions with options of a probe, air refueling receptacle and boom.
The aircraft was later marketed by EADS with 164 tonne MTOW, 5 ACTs (each carries 5770 kg of fuel, incidentally), the option of a boom or centreline hose and would have been powered by the GE CF6-80C2A8 or PW 4156A. But no probe or receptacle.
But this was all true of 'production line' aircraft. Whereas both the German A310MRTT and the Canadian CC150T are modified versions of aircraft already in service. The customers' mission requirements could be met with 4 ACTs and there was consequently no need to increase the MTOW or uprate the engines. Few military customers these days opt to pay for surplus capability, so the 157 tonne version with 4 ACTs is the version now in service with both the Luftwaffe and Canadian Forces.
However, if a customer were to acquire some 164 tonne MTOW A310 aircraft with a view to modifying them to carry 5 ACTs, that could certainly be achieved. Some of the additional AAR systems were designed with 5 ACTs in mind, so this requirement could still be met.