Multi-msn (E-10): was considered and discounted by the US a few years back with costs, integration, different operating characteristics, system fusion and most importantly COST being the driver for the block40/45 E-3G upgrade and P-8 Poseidon procurement.
RPAS: available bandwidth for comms and data relay likely to be the biggest hurdle. Perhaps the great great grandson of 5G may be a potential answer.
Dirigible: may offer greater persistence but manned or unmanned with bandwidth for unmanned yet again needing to be considered.
Smaller platform = smaller crew = potentially more limited capabilities ie smaller sensors suite with more limited range.
Satellites: geosynchronous offers persistence & wide field of regard at massive cost. Sensors with bandwidth for data/comms and counter satellite capabilities are very big questions?
Upgrade: current production line in place (for sister fleets) and most likely to be the most cost effective solution (in near term) with probable life extension beyond OOS date.
Different variant: reopening production lines (E767) won't be cheap.
Cost vs need vs another capability 'vacation'?