My guess is that it is very unlikely India will join GCAP. If the GCAP partners approved the idea in principle, negotiations with India would likely take so long that there would be nothing left for India to do development-wise by the time India joined. India does things at a different speed to other places, at least where defence is concerned.
Another GCAP partner? I could see Canada being interested in joining (for obvious political reasons). KSA is a possibility, but I wonder what contribution it could make apart from funding.
PS In India's shoes I would do what is required to put Tejas Mk2 on track and carry on with AMCA until it is ready, then keep ordering more and more Mk1 until a suitable engine for Mk2 was in production.