I don't think you should expect a new price tier between premium anytime soon. I would realistically only expect additional price tiers that are more expensive (for families for example).
A cheaper package makes no sense from the perspective of their business model, since given the current limited premium feature set, it would make the premium package unattractive for most of their customers, effectively lowering their revenue potential. Bunq's only way forward is optimizing the feature set between 'free' (goal: atttract as many new users as possible that fit the target audience of people that at some point in time are willing to minimally pay for premium), 'premium' (goal: convert as many free users to paying users as possible) and deciding which features they charge surcharges for (currently: additional cards, cash withdrawals).
I've let it sink in and for me personally I'm fine with the price of the premium package. There is merit in supporting an innovator such as Bunq and the features are pretty unique (even if I have to maintain an account at a big bank as well for traveling abroad because I cannot withdraw cash from ATMs in most cities in the world).
The breaking point for me is the limited feature set of the premium package: it does not include a second free maestro card and for me as an adult who has money to spend, for whom budgeting is necessity and not luxury and who has a relationship, the premium product offering is insufficient.
I realize they need the 8 EUR pricepoint given their business model choices, I realize they cannot offer unlimited free cash withdrawals in the SEPA given the cost, but the extra maestro card, besides the fact that it makes bunq more attractive for new segments such as couples and families, is something they can easily fix (and it would align their offering with their competitors). Offering a second free maestro card would also reinforce their argument that they want to keep the pricing model simple.
Bunq is in a transition phase: the product seems to have been built with relatively low income/young people in mind (pay for what you use, budgeting using different IBAN accounts, splitting dinner and drinks bills, shared accounts for holidays or other common activities that young people without too many responsibilities undertake).
With the introduction of the current price model, I think we can say bunq has learned that the target audience they had in mind when they envisioned the product is not big enough and that they need to attract a different audience which can afford the premium price tag. The challenge is that they've changed the price model already, but they have not changed the product a lot yet to accomodate these new segments.
So to go back to the intent of this topic: the question I believe is not what price models bunq should use. We're past that.
The question is which customer segments are willing to pay the premium price, what features need to be offered to attract them via the free subscription and what features need to be offered to get as many of them to move to a premium subscription.