Hello! I've only started using this extension this week so I apologize if some of this is already feedback you've received or if some of it is already a currently available feature.
Even though this extension is new to me, I've used a few other similar dictionary lookup extensions in the past when learning Japanese and can see a few potential points of improvements that could help overall or for the pro version.
To answer you directly - with the right features, ease of use, speed of updates etc. I would be willing to pay for a pro version. Certainly a one time payment, and possibly for a subscription depending on various factors. I realize that an extension (and accompanying mobile app etc.) can take quite a bit of development time, but it would be pretty difficult for me to justify paying more than $5 per month for a pro version, although I can't say concretely without knowing exactly what the product is and how helpful it is to me.
One thing I've noticed is that the lookup times can be a bit slow for words. I assume this is because the Dictionariez is making the request out to the appropriate API, waiting for a response, and then showing that as the results. This works well, but I do find sometimes the wait times are becoming frustrating. This is particularly true when I'm reading something outside of my comfort zone and am having to look up multiple words in every sentence. In other extensions when I was able to locally load a dictionary into the extension myself, the lookup would be quite a bit faster and that could be a good feature here. Even if it did a local lookup and got a near immediate result, and simultaneously made an API request to the other dictionaries and expanded the popup translation window when those results were returned that could be pretty cool.
I'm currently using Dictionariez while reading French, and one thing that I think would be helpful is when I look up a specific verb that is already conjugated, I often don't know exactly which conjugation is being used. Dictionariez will tell me the meaning of the base verb, which is great, but if I don't know or understand the conjugation in context then I find myself going into a new tab and Googling. If the popup could either 1.) show the conjugations for that verb that I'm looking up, or 2.) just link to an external source that shows the conjugation tables, I would find it extremely helpful. Obviously this is language dependent, but I could see it being useful in many languages that have complicated verb conjugations.
And just to add some support to features already mentioned in this thread, support for use in PDFs and a mobile version would both be really great selling features in my opinion.
I think that's it for now, I'll keep using Dictionariez and see if I come up with other ideas as well but I hope you'll find something in here useful!