Richie Porte will be 34 before any GTs start next year, and he only has two GT top 10s, one of which was the freak break in the 2010 Giro. Obviously he's long since proven himself in stage racing, as he's collected a formidable one-week palmarès, but realistically, his GT palmarès is still pretty weak. Given that, it's not surprising that many of the top teams are not going to give him a contract to be their #1 guy given that you would expect him to start declining fairly soon. The likes of Sky have their leaders of his generation - Froome, Thomas - and their young successors - Sivakov, Bernal - and likewise Movistar may have Valverde, but Quintana and Landa are five years younger than Porte, plus they have Soler and Carapaz too. Bahrain have aging leaders which may free up some space, but Nibali is only two months older than Porte and infinitely more proven in GTs as well as one day races, Bora will never need a big-time stage race leader so long as Sagan can bring them all the success they need to keep the team prominent, Mitchelton have Chaves - 5 years younger - and the Yates brothers - 7 years younger - and seem to be gravitating away from their "Aussie Aussie Aussie! Oi Oi Oi!" roots, and I'm not sure Porte would want to be well down the totem pole either. Also, Jack Haig's pretty good for the future too. No GC contender worth their salt is going to Quick Step, Lotto-NL have Roglič and just locked Bennett down, Sunweb's led by Dumoulin who's 5 years younger and they already have in Hindley and Storer two strong Australian climbing prospects, plus they have Kelderman too of course, and UAE have Aru.
That more or less leaves Porte with Trek, Katyusha (where he'll probably only be co-leader with Zakarin anyway), the French teams which I don't see as particularly likely, or places like Dimension Data and Slipstream, where aging contenders go to die or throw one last roll of the dice at the top level (and the latter have Urán too anyway). Having been very critical of Aqua Blue's team as a bunch of decent but unspectacular journeymen, that's still a level higher than Cuddles' original BMC team and not much of a step down on teams like Dimension Data and EF-Cannondale, especially as signing Porte may lure a couple of other decent level veterans to help teach the most interesting riders ABS have got, i.e. Dunbar and Pedersen and strengthen that squad a bit, especially as the team boss is smarting a bit this year from not getting the Vuelta invite he thought he deserved (but wasn't as vocal when he equally arbitrarily got that invite last year, of course) so will be keen to prevent that happening again. There aren't many ProConti teams you could argue were strong enough to support a leader like Porte at the level of calendar he would be looking for, even as a veteran, and so the ProConti teams who have the clout or the ambition to step up would seem the only realistic ones, which leaves ABS and Israel Cycling Academy, since Cofidis and Fortuneo already have their figureheads and signing Porte would be against the stated policy of Roompot.