Or perhaps a conventional sensor that is, like conventional industry, a combination of all possibilities but way worse - so it'd be like 0.2 EM 0.2 TH 0.2 AS 0.4 Geo in a 50 HS package.
This would be nice. However, I suspect it would require more work for Steve to add a new component type and doesn't solve the problem of lacking a MFC.
The easy solution to the active sensor/MFC problem is to have a conventional EM Strength similar to conventional engine tech which is available at the start of the game. This would enable active/MFC sensors and I think should only require two lines in the DB - the EM Strength 10 tech would not even need to be changed since it has TN Tech as a prereq.
Alternatively, have conventional versions of all 4 sensors that require only wealth to build.
I would love to have a game where the first stage of interplanetary exploration (and primitive warfare) is done by fighter-sized vessels.
You can actually build thermal and EM sensors without a problem, although EM requires active signals to detect in order to be worth having. I'm fine with grav sensors being TN tech as they're locked behind JP Theory anyways, so Geo sensors are the missing link (along with active + MFC).
Conventional warfare with these limits would be an interesting tactical situation since not only are only missiles possible to build as weapons, but there are no magazines. Thus you'd have squadrons of box launcher-armed fighters taking runs at each other and fleeing back to a base to reload (ordnance transfer systems exist, but again with no magazines are functionally useless. Hangars are also not possible). There would be an interesting dynamic between building for missile payload, sensor range, speed/evasion, and armour on the shipbuilding side as well as trade-offs of speed/range/power on the missile side as usual, so there's certainly some play there.
The other cool feature would be a requirement for physical supply lines due to lack of mass drivers, offering tempting targets for space piracy and such. One thing I hadn't realized though was that colonization requires conventional industry due to lack of TN Mines, but a good RP setup can give a reason to relocate those from Earth anyways.
I think it also would be very, very nice, if all conventional-tech objects will reqire no TN mineral.
I would not change ballance, still would bring much flavour to slow-start campaings.
Presently in a conventional start you can still mine TN minerals, but very slowly due to the limits of conventional industry versus TN Mines. I think keeping the TN material requirements is good to promote a little bit of resource management in terms of not outpacing your slow pace of stockpiling.