At present we have several Merchant 5 module upgrades under way--see the blog for details--but the priority for those is based on customer demand and/or sponsorship support. If there is a 4.x module that you absolutely need for your 5.x store, splitting the development cost with us can speed up releases greatly.
We also develop one-off modules for specific purposes, e.g. we have written over a dozen payment modules, including:
1st Africa Bank (UK)
Able2Buy (instant-financing, UK)
V12 (instant financing, UK)
SIPS WebPay (Thailand)
BillMeLater (instant financing)
DMD (New Zealand)
We have the largest and most experienced team of Miva Merchant developers in the world. We are also available for writing custom web applications using Ruby on Rails.