Yes, so you want to make MTA:V and/or MTA:IV? Ok.
Most modern computers can barely launch IV without problems. I mean, I have a high end computer and I can't run GTA IV due to the buggy RAGE engine which crashes me every hour and lags me. This isn't just a one-off thing, either. A lot of people have been experiencing this issue ever since the launch of IV, and you want to make a massive multi-player client for it? Uh, good luck.
You need to think, MTA devs still haven't worked SA to its limits (to my belief) yet, so how will abandoning SA to make IV be good? Mainly people play on SA as everyone can play it, and it's easy + flexible. You don't even know how hard it is to make stuff for IV...
Sure, it would be great and all, but they day isn't there yet when SA may be abandoned. I doubt it will ever be. I'm definitely playing it until the end.
If you wanna learn coding languages (C++) and create it, feel free. I'm sure they'll be people who will love to help and contribute.