I get the feeling TPG will build their network and become a...

I get the feeling TPG will build their network and become a royal pain in the ass before they buy up.
It will be cheap, it will be dodgy, but many won't care. They will use technologies like VoWifi so every TPG household will have great mobile coverage. They will use LTE-U using free unlicensed spectrum at 5.1GHz and 5.8GHz. They may also use long range Wifi products extending VOWifi coverage around their fibre nodes.
Getting tower space will be difficult. Many tower sites are leased, Despite all carriers having access to each others towers its often not economic or practical. The best place to be on a tower is the top. You get over obstacles like trees and buildings, and when the curvature of the earth kicks in. Being on top means more coverage too.
Telstra and Optus in particular are now servicing so many frequencies and technologies they are taking up or reserving for future expansion the top portion of their towers. Another carrier will probably have to sit way down the tower or build their own. Sitting down low has major issues for coverage. Building towers takes time.. about 2 years. Council approval, community engagement, public notices.. One of the reasons vodafail happened was because even once they knew they had a problem and had a budget, it still takes 2 years to make it happen.. Vodafone were 12-18 months too late, delivering a plan that actually takes 2-3 years. I think TPG will be going it alone from all that I have read. They will create the cheapest and by far the worst network in Australia. It's going to hurt, Vodafone and the MVNO's the most... players at the bottom end of the market.
If there is a merger with TPG it will happen after they have bought market share, and need to improve their network in my opinion. It has always been thought that Vodafone and TPG should get together, but with TPG having megalomaniac ambitions in Singapore too, one wonders if Optus will make an offer in a few years and get rid of two thorns in its side..