Personally, I hope the new mobs within Dynamis aren't level 90+. Part of that is because, yes, it will be some element of chasing old gear, but also the more difficult you make them, the more likely you risk forcing people to seek larger groups where the inequities that entails can surface.

That said, incentive to enter even if you have all your relic gear needs to be present. Maybe we'll be blown away by the tweak of relic weapon requirements and it'd be something like 5000 currency total to complete one, but that's being optimistic. Realistically, we could anticipate trials involving the zones, potential new drops, or even the currency itself. I know I've mused over the possibility of combining all our various AF combinations into a single piece, and that could very well be possible if SE deemed a means to +2 everything or something as a pre-req, but we obviously know it won't happen until it does, if ever.

Besides, there's still some element of the random number generator involved that makes Dynamis less appealing than Abyssea on the whole. Nobody wants 20 BST when you need at least one RDM, BLM, or whatever. I guess if they keep job-specific NMs with higher rates and more readily popped, you can kind of get around that, but it also risks congestion issues just like Abyssea's Gukumatz, Amun, and so on if it's found that something in particular is the sole source of something good.

I guess it could be interesting if they added another currency like cruor, ichor, allied notes, and so on that you'd get after killing mobs in the zone. It'd allow them to slip later level options into the reward scheme without presenting as much of a risk of saturation. Could even tie Relic trials into it much like how some magian weapons needed things from Dominion and Bastion.