I want an SDK because I want to create content for the new DTG Flight Sim...
I use the FSX SDK to create content for FSX, specifically aircraft models. If they're good enough I'd like to release these models to the FS community freeware libraries.
My workflow looks like a lot of jumping around from plans, photos, books and magazine articles until I've got a proper idea of what I'm trying to build, then a lot of work modelling, mapping, animating a particular model with a great deal of trial exports to see what the thing looks like in FS in collaboration with a painter. Coding will come later.
Things that annoy me about the FSX SDK are although it's the best SDK produced for any version of FS, it's targeted squarely at people who already know what they're doing; although quite comprehensive, it's rather haphazardly organised and there are whole areas like flight modelling which are simply not documented. Someone reading the FSX SDK is met with a raft of what really cool things Simconnect now includes and talk of EventIDs and Simulation variables. The beginning modeller could do with a plain guide to starting out instead of burying New Aircraft Procedures away in Environment Kit/Modelling SDK or making airport placement as clear as mud. Nobody's pretending this can be made primary-school simple, but some thought to direct the new developer towards a useful workflow would be a plus.
The FSX flight modelling is still not documented except by third parties: that cannot be acceptable for any future flight sim. This isn't rocket science, but it runs pretty close and to leave us to guesswork, random trials or resorting to voodoo is in nobody's interests. BTW, a properly developed turboprop section of the flight modelling would be a big plus.
A scripting tool or editor which checked coding syntax and typecasting in animations or systems would be a real help compared to FSX and avoid long frustrations from a missing or surplus space/parenthesis or catch us trying to divide a boolean by 7 and couldn't get it to work?
I used to use Gmax for modelling and now use 3ds Max: it's an old version but as a hobbyist it doesn't cost me anything more to keep on using it. If a DTG SDK toolset only supports the latest version of 3ds Max (now 2017) you will kill off the hobbyist potential immediately: it costs £186 per user per month. Gmax is no longer an option, so you have to support a lower-cost or freeware modelling tool to bring in the learners and small-time freeware developers who will produce the future PMDGs and Aerosofts of the FS world. For graphics I use Photoshop CS4 – 'nuff said. None of these are easy to learn, but we're trying to simulate aviation, not join-the-dots drawing. Simple tools just can't produce the standard of work the simmers are looking for.
You'll get many pleas to make your SDK as simple and easy as possible – please instead try to make it as comprehensive as possible, updated as thoroughly as possible when new features are introduced and consider date-stamping the entries so we know what's been changed and when.
A really good SDK makes third-party add-ons much easier. Better third-party add-ons attract more customers. More customers buy more copies of the sim. That's business.
And thank you for asking! (plus there are lots of actual developers at FSDeveloper.com – these are the people you should be asking)