Future development of the Interstate System, under current processes, will be completely dependent upon state and local activities toward establishing corridors as deemed necessary by their progenitors and, over time, amassing the necessary funds for construction. It would have been nice if there were an orderly and cohesive nationwide progress -- as indicated historically by the 1968 nationwide addition program -- 10 years after the system was being "gelled" into its first iteration and ostensibly prompted by (1) population growth and (2) demographic changes. True, some of the 1500 miles of '68 additions were clearly politically motivated (I-72 in IL and I-88 in NY as examples), but most of the others were logical additions to the system (I-15 extension to San Diego, I-75 likewise to greater Miami, I-40 east through Raleigh). This, IMO, should have remained the expansion process: on a regular basis (10 years seemed reasonble back then -- and still does today) states/regions submit corridors based upon their perceived need; they're vetted at the USDOT level (with FHWA and AASHTO chiming in, of course), and the most warranted corridors added to the chargeable system. Alas, after '73 it was not meant to be with the Nixonian shift to block grants and the Feds relegated to the role of determining standards. The goal of those who formulated this change was, simply, to abrogate much of LBJ's "Great Society" undertakings -- which undoubtedly did result in the expansion of the federal bureaucracy, for better or worse -- and to ensure that such major programs didn't reemerge. However, the end result was that any number of worthy activities and goals were swept into oblivion along with those that particularly irked those formulators; at that point, any expansion of the Interstate system -- essentially inevitable with the demographic changes since the early '70's -- was the parvenu of those at the state & local level who were most competent at manipulating the various levels of governance into manifested action. Some of these were actually worthy projects that did address demographics and/or the shortcomings of the original system configuration (I-49, I-22, possibly the southern half of I-39); some were obviously local "pork" or manifestations of longstanding resentment about being left out of the network in the past (I-99, I-73/74, the new I-87, the I-14 proposal) -- and some were Frankenroads cobbled up to get national attention for "serialized" local projects (I-69, obviously). But, to reiterate a cliche': it is what it is. Since 1991 the various comprehensive national infrastructure legislative efforts (ISTEA and its successors) have provided a convenient path for the establishment of new Interstate corridors (albeit lacking any specific funding source): the High Priority Corridor, which can be "tweaked" with a little legislative moxie to designate a specific corridor as a future Interstate, often with a number attached. That's how we got every 2di designated after 1992 (with the exception of I-2 -- but even that was "piggybacked" as an adjunct connector to extant HPC-originated corridors) : the promise of the maximum 80% federal funding (albeit without guarantee) -- of course dependent upon Congressional apportionment plus state/local-originated matching funds, which has proved to be a pretty high bar to overcome. There's a pathway for future Interstate development -- but the present one has just created an ever-increasing "backlog" of possible future projects. Right now promoters of new Interstates can take a slight measure of solace in the fact that the process exists and has produced corridors (the viability of which can vary widely) -- but those who decry any incursion of more freeways into urban areas can also be thankful that most of the proposals to date have been rural/intercity in nature, with very little developmental activity in urban areas (OK, NC's the exception, as it is with developmental policy in general). Otherwise, no major parties have gone out of their way (some of our posters excepted!) to plan and deploy new urban Interstates; the writing seems to be indelibly on the wall for that type of activity.
What I'd expect to see in the future are (a) connectors to and between areas unserved by the present system, including efficiency enhancers, (b) "relief" corridors for present overused portions of the Interstate network, and (c) "spot" projects intended to alleviate localized discrepancies. Most of the proposed TX corridors, including the peripatetic "Port-to-Plains", fit into category (a) -- certainly not much of (b), considering the sparse use of I-10 west of San Antonio!; the midsection (Shreveport-Memphis) of I-69 is more (b) than anything else, considering the traffic on I-30 and especially I-40 Little Rock-Memphis. (C) projects crop up all over the place; I-269 and I-840 in TN (and MS in the case of the former) could be considered both (b) and (c); 269 as a "real" bypass loop considering the urban inundation of I-240, 840 relieving movements in and around Nashville. But DOT's need to be very careful to configure the terminal miles of any of these nascent intercity corridors so as not to promote sprawl (i.e., avoid the temptation to take developer $$ to put interchanges along the routes).
How all this will work out in the future is anyone's guess. Those who want to do a major societal reconfiguration will likely be disappointed that what will likely be built won't do anything to suppress the individualistic tendencies that drive what passes for our "culture" -- but it also won't be a problem to those who want to create urban "reservations" relatively free from commercial considerations -- no one is proposing to raze major city areas for massive new freeway projects (teardowns are another story altogether!). But despite the wishes of some, this will be a commercially based societal structure for at least the near future, geared toward both the individual and groups forming from social instinct (and occasionally common duress!) rather than a "forced march" toward a predetermined standard.