@wawrzon
I am sure there are ways of developing for both Classic AmigaOS and OS 4.1 mutually in terms of Workbench applications and libraries. This helps bring the development costs down and gives much needed vital investment back into Classic at the same time.
but as far as i have understood you, you were talking about bringing os3 to the state on pair with os4 in order to enhance common development. you can do that without access to os4 and its sources, but then it would mean reverse engineering and reimplementation, like in case of mui4 for os4. this causes an additional (costly, as you yourself admit) effort, while aros provides already good amiga compatibility while introducing technical features on pair or probably in many cases even better than os4 itself. with aros you can actually maintain libraries for different architectures automatically side by side, without that much hassle about different interfaces, like those used on os4.
dont get me wrong, im not trying to talk you into aros, but i dont understand how and why do you want to handle parallel development on what you call "classics" and os4, if experience with existing efforts in this direction apparently prove problems, jut to name magellan or personal paint.
I have learned recently how costly real software development is, so developing FPGA solutions for Classic is one important step, then paying for software to be written for them is another expensive project. I remain positive though that solutions can be found.
if you want to develop native applications from the scratch then probably. but if you just port things over, there is already a number of enthusiasts doing it for free and the sake of it. im positive, that as soon as they may lay their hands on fpga hardware, it will boost their interest.
on the other hand i just dont see a possibility to develop huge and complex native software packages for affordable money. people may do it out of own interest, like for instance hd-rec, but i doubt you are able to provide enough funds to commission software you wish for and ensure its maintenance.
again i see here only realistic possibility to motivate people in certain directions, towards simple detailed goals by means like contributing to the bounties.