I am a little bit surprised that there's not an especially good red deck in Pioneer. Feels like red aggro has been at least Tier 2 in Standard through the whole range of legal sets, even the best deck at times.
Thoughtseize seems like a card that is both driving the Pioneer metagame to a bad, same-y place and keeping it from becoming a bigger mess. It being the best card in the format and basically the only good cheap interaction means you're strongly incentivized to play it yourself and that you can't play something that just folds to it (hence all of the aggro and ramp/midrange). Without it though, I imagine things would just get more linear.
The aggro/ramp/midrange metagame did feel inevitable from the announcement though - threats have outpaced answers so dramatically since like Innistrad/RTR. I don't think it's realistically possible for Wizards to fix this in the short term either. Printing the quality and quantity of answers for Pioneer without making Standard a control-only hellhole would be impossible without Pioneer Horizons, and banning enough busted threats out of Pioneer would probably turn a bunch of people off of it.