I think the efficiency of the Xplorer train sets could be boosted by the addition of a converted XFs (XPT 2nd class coaches) to Xplorer trailers. In terms of traction the Xplorers have enough grunt. Hotel power is another issue, but turning down the AC and/or adding a few solar panels should make this possible without having to install generation into the trailers.
That would boost the size of 7 of the 8 Explorer sets from 150 to 220 seats, and make them suitable for some of the lighter loaded XPT routes, while making them cheaper to operate than a 4 car Xplorer now.
The second idea is to merge some of the Endeavour sets with some of the Xplorer sets, to create mid-range DMUs with a mix of seating. The result should be a long 4 car version with ~270 seats (similar to a 6 car XPT) and a buffet, and a short version with 150 seats and some vending machines. I don't fancy sitting on an Endeavour seat for 10 hours (though it's better than on a bus), but that's not what they are for. They are for shorter haul PAX and/or overflow. The short version would be ideal for the Canberra route and the Bathurst train.
For a "cheap" replacement, I think a V'Locity style DMU driving cab unit with - the critical feature - a mating compatible with the XPT coaches and enough grunt to lug a trailer. That would produce a train with seating for ~250/300 (depending on how much space is given over to meal service and luggage) for the cost of a 2 car DMU, basically enough to cover any of the XPT routes now.
The second tranche of the order would be for non-driving cabs to deliver a cost effective 2 car train, and a third tranche to eventually replace the XPT coaches as trailers.
Initially that'd mean a typical (initial) capital outlay of ~$12mil per 4 car train set, rather than $24mil. I think these trains (with a crew of 3) would cost ~$750/hr to operate (vs $1450 for an XPT) and thus deliver ~$0.035-$0.04 per seat km (ie on a part with busses).