Deflationary, inflationary, both are undesirable in a currency.

Can't we talk about having a STABLE currency, and what it would take to keep a currency reasonably stable?

Prices, ideally, should neither increase, nor should they decrease. The ideal for any currency is stability.

Bitcoin is a great step forward in the direction of having a stable currency, but it does not seem that its creation algorithm is directed towards obtaining stability. We will have gradually increasing difficulty of mining, until a few years from now, mining will practically come to a halt.

Now if bitcoin were to serve a substantially stable market, meaning there is a community of bitcoin users that does not either substantially increase in numbers nor decrease, the currency could be said to be stable in the long term.

I do have a feeling though, that the long term intention of the creators and the early adopters of bitcoin is to expand the user base to hundreds of millions, perhaps even to billions of users.

Since stability of a currency is a function of demand (user base) and availability (fixed in our case after the target amount of bitcoins programmed by its creator has been reached) in a long term view, this means that bitcoin will very likely be a deflationary currency, i.e. that prices, as expressed in bitcoin, will be continually falling, as the user base grows.

For stability, the target amount of bitcoins should be ajustable - preferably by some automatism - to follow the growth of the user base.